Disclaimer: If you recognize it, I don't own it.

Marco frowned, watching Ace and Luffy sleep against the scaled side of their oldest brother. He was grateful for Sabo's help, of course, without Sabo Ace would have been dead for sure, but there was something bothering him. When Sabo had arrived there had been a number of men in cloaks who Marco had assumed to be Sabo's crew, so where were they now. As if reading his son's mind, Whitebeard leaned closer to the trio to speak with Sabo.
"I thought you had some others with you. What happened to the rest of your lot?" Whitebeard asked, keeping his voice down to avoid waking the sleeping pair. Sabo raised his head and looked over the rail at the ocean for a moment before turning back to Whitebeard.
"Nothing to worry about. My crew jus' went ta fetch the ship, they'll catch up with us any time now." He said, looking back over the rail again. "In fact, I do believe that's them now." Marco looked out at the ocean and froze, eyes wide. Not five miles out was what looked like a tear in the air itself, as if some great beast had slashed it apart. Sabo just chuckled and started nudging Ace and Luffy with his snout. "Wake up, you two. You're not gonna wanna miss this."
"Ng, wha…?" Ace sat up slowly and stretched, looking around in a daze, whereas Luffy just wrapped his arms tighter around Ace's waist and snuggled closer. Sabo stood, leaving Luffy's head to hit the floor and wake the boy up, and shifted to his hybrid form: mostly human but with the deer-like feet, reptilian tail, long ears and back-sweeping antlers of his kirin form. As Marco watched a fair-sized ship came through the tear, the ship a shade of blue that would blend in to the ocean easily with two light-blue sails and what looked like a kirin's head and front legs for a figurehead.
"Whoa, cool! Who're they? Do you know them, Sabo? Are they friends of yours?" Luffy asked, jumping up excitedly and grabbing the rail, leaning over it to get a closer look. Sabo laughed and grabbed the back of Luffy's vest with one hand to keep him from falling while reaching over with his other to close Ace's mouth, which had fallen open upon seeing the tear.
"Yes, Luffy. That's my crew, the King's Guard Pirates. Our ship is called Dream's Bounty and the tear in space is the handiwork of my nakama, we call'm Rift, fer obvious reasons." Sabo said. "I'm thinkin' you'll especially wanna meet Rift, an' possibly ol' Gem, too." He told Whitebeard, grinning.
"I take it this 'Rift' was the one who got me out of that tight spot, then." Whitebeard said, thinking back to the cloaked stranger who had appeared while he was facing off Blackbeard's crew and clawed a rift similar to the one the Dream's Bounty had sailed through just now between them, catching the enemy crew in the rift which closed behind them.
"Captain, you still over there?" A low voice called over, five cloaked figures appearing at the rail.
"I'm hear, Gem. Come on over." Sabo called back. "An' bring Rift with ya. It's about time we returned 'im, I think." No sooner had Sabo said these words then a new rift appeared, this one opening right behind Marco who grunted when he was suddenly tackled to the deck by the cloaked form who emerged before it closed, causing most of the Whitebeard Pirates to reach for their weapons.
"Excitable idiot." The first cloaked man, identified as Gem, yelled as he jumped over. "They just left a war and now you're jumping them. Do you want to end up dead again?" Marco growled at feeling sharp claws on his abdomen before Rift let go and climbed to his feet, laughing and disturbingly familiar laugh.
"Sorry, sorry, couldn't help myself." Rift laughed, offering his hand to help Marco up. Marco accepted, letting Rift yank him back to his feet, before ducking his head to try to look under Rift's hood, frowning when all he saw were shadows and a smile that he knew he'd seen before.
"You want to explain yourselves?" Whitebeard growled, motioning Marco closer to him. Marco reluctantly stepped away from the stranger to stand next to his father, though he never took his eyes off him.
"My apologies, Whitebeard Sir, Rift's always been a touch on the excitable side. No harm was intended, I assure ya." Sabo said, bowing slightly. "See, we picked him up a couple months ago after he was attacked by a particularly vicious individual, a rescue that was only pulled off so flawlessly due to my dear friend here. Gemini, Gem fer short, has the ability to duplicate things. Sadly, he can't duplicate life, just its vessels."
"Does this have a point?" Izo asked when Sabo paused for a breath.
"Yes, in fact. I can see you're not in the mood fer patience, though, so I'll skip the longwinded braggin'. The short of it is that we found Rift half bled-out, Gem duplicated his body to leave what would appear ta be a corpse ta throw his would-be killer off the trail, and we been waitin' for a chance ta nail ol' Blackie ever since. Course, if we'da known the trouble Ace would get into chasin' after Blackie we woulda tried a bit harder. Ah, now you're catchin' on, ain'tcha." Sabo said, noting the way a few nearby pirates had stiffened and started trying to discreetly look under Rift's hood. Whitebeard looked at Rift, searching for any clue that his half-formed suspicion was true before giving Sabo a glare.
"Boy, if you're messing with us…." He trailed off as Rift reached up to grab his cloak, pulling it away and looking up at Whitebeard with a grin.
"Hey, Pops. It's good to be home." Hearing a thud, Thatch turned and chuckled. "Maybe we should take it easy on the shocks." He said, nudging the again fainted Ace with his foot before turning back to his father just in time to see both Whitebeard and Marco join Ace in unconsciousness.

END

Yes, Thatch is alive too. Basic explanation is Sabo had been checking up on his brothers without them noticing for years and, on one of his trips to check on Ace, found Thatch clinging to life and decided to save him on the condition that Thatch join his crew for a short while. Gemini duplicated Thatch's body so Blackbeard wouldn't realize he'd failed and possibly go after Thatch again for any reason and left the body there to be found. Thatch wanted to contact Whitebeard and tell him he was okay, but Sabo didn't want to risk any other traitors within the crew finding out and alerting Blackbeard. Thatch later ate a Devil Fruit, the Rift-Rift Fruit, that gave him claws that can cut through space (and nothing else no matter how sharp they feel) and make travel easier. He does need to know where he's going to open a rift, though. He opened the rift to the Moby Dick with coordinates sent by Sabo. Gemini's fruit is called the Double-Double Fruit. Gemini can't duplicate the abilities of Devil Fruits, though he can make physical duplicates that look and taste like Devil Fruits to use as decoys.

Edit: Fixed a few mistakes. 9/6/12.