I've edited this, just changed some of the mistakes I found. Update should be coming soon.
Disclaimer: I don't own anything or anyone in this except for Sierra.
"Get the hell away from me!" Sierra screamed, backing up and trying to crawl into the wall itself "Just get the hell away!" she plunged a hand into her robes and pulled out her wand, pointing it at Sirius with a shaking hand.
"Sierra." Sirius said slowly, backing up and keeping a wary eye on her wand "Come on. It's just me, you're alright."
"Fuck you." Sierra growled, her eyes narrowing at him and then moving to look at the others, who were staring at her in shock "Who the hell are you people!" she screamed, trying to back further away.
"Careful Sirius." Tonks muttered "She's lost it. I don't think she even knows who she is."
"I think I can see that." Sirius replied, placing his hands on the ground slowly to steady himself "Sierra, come on you psycho girl, talk to me."
"Get. The. Hell. Away." Sierra spat at him, sparks spitting dangerously from her wand "I know you. You want to kill me. Just like all the others!"
"Sierra-" Sirius started but she cut him with a animalistic growl
"Get the fuck away!" she screamed "You killed Jake, I knew you'd be back for me. Well I ain't letting you take me before I get my pay back!" she lunged forwards and Sirius moved back, dodging his sister as he stared at her wide-eyed. Sierra turned to look at him, gasping for breath and glaring. The others had their wands aimed at her, but Sirius shook his head and gestured for them to lower them.
"Sierra." He said "Calm down. Please. You're safe, just –"
"Fuck you!" Sierra screamed, she jumped forwards, transforming into the wolf as she did so. Sirius met her mid-jump as the bear-like dog, sending them both to the ground in a flurry of claws, teeth and angry barks.
"Everyone get back." Moody growled, flinging out his arm and stepping back, making everyone else do the same. They kept there eyes on the fighting twins as they battled, biting and ripping at each other.
"We need to do something." Tonks said slowly, pale with shock at the scene before them "They're going to murder one another at this rate." The animals were moving to quickly for them to assess the damage they were doing to each other, they could only just hear, over the barks and growls, the whimpers that came each time a clawed paw or set of teeth found a venerable part of its opponent.
"Alastor." Arthur Weasley said, the two exchanged a glance and nodded to each other, drawing their wands. Whatever they were about to do was stopped when the twins broke away from each other and stared at each other for a moment, both were equally bloody, Sirius had a large gash across his muzzle and several fairly deep cuts on his back along with many minor ones. Sierra had a cut across one eye, blood blinding her vision and one paw was held above the ground. She whimpered several times before lowering herself to the ground, lying her head on her uninjured paw and gazing sorrowfully up at the others.
"You think they've finished?" Bill asked
"I reckon so." Moody replied as Sirius changed back and stood slowly, grasping the kitchen table for support. He wiped at the bloodied gash on his nose and winced, looking down at the wolf as Remus and Arthur stepped forwards to steady him.
"Sierra?" he asked. The wolf looked at him for a moment before whimpering and turning her head, embarrassed by what she'd done. "You know who I am?" she looked at him again for a second before turning her head and wagging her tail slightly. Sirius smiled and bent in front of her, placing a hand comfortingly on her head
"Going to change back?" he asked "Or will you stay as a wolf for the rest of your life. Not sure about getting you raw meat everyday, but it'd save time in cooking." In an instant Sierra was transformed back, pushing herself up from the ground slowly and pushing her brother weakly with her good hand before collapsing against the wall.
"Sorry." She whispered, tears running from her eyes as she shuddered. Sirius nodded and carefully sat beside her, wincing in pain as he stretched his arm around her shoulders and hugged her
"I know," he said "Just proves you really are psycho though. Knew it from the start. Sierra glared and punched him weakly, looking up at the worried faces of their friends before she slipped into unconsciousness.
Sirius felt his sister go limp against him and turned his head to look at her
"Sierra?" he said worriedly, gently shaking her. She didn't reply and he bit his lip, turning his gaze to Molly as dropped beside them
"She's alright I think." She told him, gently putting fingers to Sierra's neck "She's worn herself out. We need to get you both cleaned up before you lose to much blood though." She shot a worried look at Sirius before lifting up Sierra, grateful that the younger woman barely weighed anything. Sierra whimpered as she was moved but stayed still. Remus offered a hand to Sirius and pulled him to his feet as Molly carried his twin past.
"She'll be fine." He promise with a thin smile "Come on, Molly will kill us all if we don't sort out your cuts."