Percy is definitely her mothers daughter. A fact which Hazel and Frank are coming to realise. Also, she has no care for if she dies or not.
Replies:
Quip Scarlet: Percy just adopts a small army of baby siblings. And the best this is we all know Sally would just take one look at anyone Percy presents to her and be like 'Oh, you want a new sibling, come here my new child'.
Undeath9087: Ella'll be happy but I feel like Percy's just there like 'Eh, I just kinda murdered my brother. Oh well he was a dick.' And I must say your suggestion is interesting. Though I wouldn't say Alabaster is going to kidnap her exactly. What will happen is more complicated.
JjlovesPJO: Percy is adorable with anyone who she considers to be a potential baby sibling and that officially includes Ella lol.
The Official Clarisse: Just imagine Percy seeing her mom again 'Hey, hey mom. Meet Ella and Hazel. They're my new sisters okay?'.
Guest: Oh she knows she knows it she just don't know how yet, which is fun. And yes the demigods do have phones, but Percy doesn't have one at this stage and they won't really have much time- if anything the reunion is more complicated because how the fuck do you bring up 'hey, so I'm not dead now!' it's definitely not something you can do over the phone.
Guest: I may post the KC au's first chapter soon but I want to be sure I won't get stuck so I want to have a couple more chapters before I start publishing that one.

Percy wrapped her arms around Ella, careful not to touch her back, "Hey- hey it's okay sweetie." she spoke gently, soothingly.

"You swore to protect Ella-" the harpies face was pressed into Percy's shoulder, "No one's ever wanted to protect Ella before."

"Then they're all dummies." Percy felt her face flushing- Hazel looked almost like she wanted to coo at the scene- and Frank looked like he was tempted to laugh.

"Is it just me or is she adopting a harpy?" she heard Frank's whisper.

"I think she is." Hazel whispered back as Percy soothed her fingers through Ella's hair, which made Percy give her friends a dirty look over Ella's shoulder and poke her tongue out, which was clearly the mature thing to do here.

"Why don't you guys do something helpful, Frank do you have our first aid supplies?"

Frank's eyes widened before he nodded, "Oh! Of course." he slipped his bag off of his back and opened it up, pulling out a thermos flask filled with nectar.

He moved closer, holding it up so that Ella would be able to see, "This is nectar Ella. It's kinda magical. If you let us pour it on your wound it should help it at least stop hurting."

And Ella pressed closer to Percy, glancing up before, almost whimpering as she pressed her face into Percy's shoulder again.

Percy felt a rush of protectiveness, brushing her fingers through Ella's choppy red hair. "Hey- hey it's gonna be okay." her voice was soothing, "Can you let us help you?" and Ella's nod was just a small jerky movement but Percy felt it with the way that the harpy had her face pressed into Percy's shoulder. And Percy hummed softly, keeping Ella's face pressed into her shoulder as Frank carefully poured the nectar onto her back, and Percy felt a rush of relief as the wound started to close.

"See." Hazel's voice was almost as gentle as Percy's "That's better isn't it?"

And Ella shifted slightly, lifting her head away from Percy's shoulder, though she was still curled close to Percy. "Ella feels better." she said after a moment.
"Phineas is bad. And weed whackers. And cheese. Stay away. Please. Don't want you hurt too."

That made Percy give a very faint smile, tucking a lock of red hair behind Ella's ear. "That's really sweet." Percy's voice was gentle. "But I can look after myself sweetie. We don't like him either. And like I said we won't let him hurt you again. But we need your help Ella. We need something from him so we need to figure out how to trick him. And you've been around him for so long that I'm sure you harpies know him better than anyone. Is there any way we can trick him?"

"N-no." Ella said. "Tricks are for kids. 50 Tricks to Teach Your Dog, by Sophie Collins, call number six-three-six—"

"Okay, Ella." Hazel spoke in a soothing voice, like she was trying to calm a horse. "But does Phineas have any weaknesses?"

"Blind. He's blind."

Frank rolled his eyes, which made Percy shoot him a warning look- and to give him credit Frank looked guilty a moment later, but Hazel continued patiently,
"Right. Besides that?"

"Chance," she said. "Games of chance. Two to one. Bad odds. Call or fold."

Percy's spirits rose. "You mean he's a gambler?"

"Phineas s-sees big things. Prophecies. Fates. God stuff. Not small stuff. Random. Exciting. And he is blind."

Frank rubbed his chin. "Any idea what she means?"

And Percy watched Ella picking at her burlap dress, her mind racing as she thought quickly- and dear gods but the harpy was definitely smart.

"I get it I think." Percy let out a slow breath, "Phineas sees the future right? He knows tons of important events but he can't see small things like random occurrences, spontaneous games of chance. Which makes gambling exciting for him because he doesn't know what'll happen. If we could tempt him into making a bet with us we could get what we want and protect Ella."

Hazel nodded slowly, "And if he loses he has to tell us where Thanatos is. But what do we have to wager? And what kind of game do we play?"

"It has to be something simple with high stakes. That'll be our best chance of hooking him. Something with two choices maybe. One you live, one you die. And the prize needs to be something Phineas wants. Besides Ella obviously. She's off the table."

"Sight," Ella muttered. "Sight is good for blind men. Healing…nope, nope. Gaea won't do that for Phineas. Gaea keeps Phineas b-blind, dependent on Gaea. Yep."

Frank and Percy exchanged a meaningful look. "Gorgon's blood," they said simultaneously, Percy's lips spreading into a grin.

"What?" Hazel asked.

Frank brought out the two ceramic vials he'd retrieved from the Little Tiber. "Ella's a genius," he said. "Unless we die."

"Don't worry about that," Percy said. "I've got a plan."

"What plan?" Frank frowned as Percy straightened up. "Why do I feel like we won't like it?"

"Because you won't." Percy kept her arms wrapped around Ella, "Now let's go and see Phineas. And no I won't be telling you my plan."

Percy could feel the looks Hazel and Frank kept shooting at her as they headed back to the food truck parking lot.

He was sat on his picnic bench with his bunny slippers propped up, eating a plate of greasy shish kebab. His weed whacker was at his side. His bathrobe was smeared with barbecue sauce.

"Welcome back!" he called cheerfully. "I hear the flutter of nervous little wings. You've brought me my harpy?"

"She's here," Percy said. "But she's not yours." her voice was like ice. "I'm afraid I'm not helping you here, sibling or not."

Phineas sucked the grease off his fingers. His milky eyes seemed fixed on a point just above Percy's head. "I see…Well, actually, I'm blind, so I don't see. Have you come to kill me, then? If so, good luck completing your quest."

"I've come to gamble."

The old man's mouth twitched. He put down his shishkebab and leaned toward Percy. "A gamble…how interesting. Information in exchange for the harpy? Winner take all?"

"No," Percy said. "The harpy isn't part of the deal."

Phineas laughed. "Really? Perhaps you don't understand her value."

"She's a person," Percy said. "She isn't for sale. People aren't prizes to be won."

"Oh, please! You're from the Roman camp, aren't you? Rome was built on slavery. Don't get all high and mighty with me. Besides, she isn't even human.
She's a monster. A wind spirit. A minion of Jupiter."

Ella squawked- she'd been nervous enough, even curled into Percy's side with Percy's arm wrapped protectively around her, now she was starting to panic again, backing away, muttering, "'Jupiter. Hydrogen and helium. Sixty-three satellites.' No minions. Nope." and Percy shifted, letting Ella out from under her arm, "Hazel-" and Hazel moved forward quickly, wrapping an arm around Ella- the harpy didn't seem too panicked with Hazel at least- she was more scared of
Frank than the two girls.

And Frank moved to, shifting to Percy's side, holding his spear ready as if he expected Phineas to attack them.

Percy held out her hand to Frank, and he pressed the vials into Percy's hand, and she curled her fingers around them. "I have a different wager for you. We've got two flasks of gorgon's blood. One kills. One heals. They look exactly the same and even we don't know which is which. If you choose the right one it could cure you of your blindness."

Phineas held out his hands eagerly. "Let me feel them. Let me smell them."

"Not so fast," Percy said. "First you agree to the terms."

"Terms…" Phineas was breathing shallowly. Percy could tell he was hungry to take the offer. "Prophecy and sight... I'd be unstoppable. I could own this city. I'd build my palace here, surrounded by food trucks. I could capture that harpy myself!"

"N-noo," Ella said nervously. "Nope, nope, nope."

A villainous laugh is hard to pull off when you're wearing pink bunny slippers, but Phineas gave it his best shot. "Very well, demigod. What are your terms?"

"You get to choose a vial," Percy said. "No uncorking, no sniffing before you decide."

"That's not fair! I'm blind."

"And I don't have your sense of smell," Percy countered. "You can hold the vials. And I'll swear on the River Styx that they look identical. They're exactly what I told you: gorgon's blood, one vial from the left side of the monster, one from the right. And I swear that none of us knows which is which."

Percy looked back at Hazel. "Uh, you're our Underworld expert. With all this weird stuff going on with Death, is an oath on the River Styx still binding?" if it wasn't she'd feel kind of bad for swearing to Ella earlier but- well it had been the intentions behind that promise that mattered.

"Yes," she said, without hesitation. "To break such a vow…Well, just don't do it. There are worse things than death."

Phineas stroked his beard. "So I choose which vial to drink. You have to drink the other one. We swear to drink at the same time."

"No!" Hazel's eyes widened as it truly clicked what Percy was planning, "Percy you can't it's too dangerous-"

"If I die I die." Percy said quickly, "I know what I'm doing- I'll go to Elysium again- either way it goes I- I'll be fine-"

"But we need you-" Frank looked panicked, "You can't be that crazy-"

"I feel like people have said that to me a lot." Percy gave a grin despite herself, "I don't care either way." and she wasn't lying- she was exhausted, after everything that happened, the lack of her memories- the horrible hints about her life? It would probably be easier for her to be dead- and she'd already died once hadn't she? Maybe this was the part she was supposed to play? And- if she died she'd just go back to Elysium, which was supposed to be good. Her life was a big price sure but- they could do it without her. Besides, if she was right then she wouldn't be the one who died. After all. Gaea had told her she had plans for her.

"Don't try and talk the girl out of it." Phineas was leaning forward, "My little sister knows her own mind after all." he looked almost gleeful, "So, this bet, the loser dies obviously. That kind of poison would probably keep even me from coming back to life for a long time, at least. My essence would be scattered and degraded. So I'm risking quite a lot."

"But if you win, you get everything," Percy said. "If I die, my friends will swear to leave you in peace and not take revenge. You'd have your sight back, which even Gaea won't give you."

"Percy!" Hazel's voice was horrified, "Are you crazy?"

"Again, yes, yes I am. Now shut up and let me do this okay?" and she watched Phineas' expression sour at her jab about Gaea not giving him his sight- she'd definitely struck a nerve. That was good.

"If I lose," the old man said, "I'll be dead, unable to give you information. How does that help you?"

"You write down the location of Alcyoneus' lair ahead of time. You keep it to yourself but you swear on the river Styx that it's specific and accurate. You also have to swear that if you lose and die, the harpies will be released from their curse." Percy had thought this through- and she knew she was going to get yelled at after this by her friends but she didn't care. She'd do whatever it too. What did her life matter when she was already dead- when her loved ones had probably already moved on.

"Those are high stakes," Phineas grumbled. "You face death, Percy Jackson. Wouldn't it be simpler just to hand over the harpy?"

"I would genuinely rather die."

Phineas smiled slowly. "So you are starting to understand her worth. Once I have my sight, I'll capture her myself, you know. Whoever controls that harpy…well, I was a king once. This gamble could make me a king again."

"You're getting ahead of yourself," Percy said. "Do we have a deal?"

Phineas tapped his nose thoughtfully. "I can't foresee the outcome. Annoying how that works. A completely unexpected gamble…it makes the future cloudy. But I can tell you this, Percy Jackson—a bit of free advice. If you survive today, you're not going to like your future. A big sacrifice is coming, and you won't have the courage to make it. That will cost you dearly. It will cost the world dearly. It might be easier if you just choose the poison."

Percy felt slightly sick at that- her mouth tasted sour- whatever was going to happen was hard. It was always hard. She felt like she'd already paid the ultimate sacrifice- she'd died once hadn't she? But- it would get worse. She was sure it would. But she had to do it. She had to help take down Gaea.