James's twin sister? (Marauders Era)

Hogwarts

*Sirius's P.O.V.*

"Padfoot are you alright you spent the whole ride staring at Alissa?" Prongs asks and I nod, "Sorry Prongs, it's just that there's something about her that catches my attention you know?" I say and he shakes his head, I sigh and look outside, ever since this summer I've been living with him. It's nice.

Suddenly the door opens and Regulus appears there "You know Sirius, mother was so pissed you left. She burnt your picture from the wall. You are officially not a member of the Black family. You're just a filthy blood traitor." he says I get up and take one step forward and suddenly he's on the floor and there is someone punching and hitting him, Snivellous appears and pulls the person back, "Alissa calm down." Snivellous says the person simply stopped moving and kind of growled "I am calm Severus, this idiot just needs to learn some manners, because no one talks shit about my friends and gets away with it." 

She walks into the compartment and closes the door before sitting down and leaning against Moony, I glare at him and let out a low growl, this seems to not only catch the boys attention but also hers. She looks around frowning before leaning against Moony again, she quickly falls asleep with her head on his lap. "Padfoot why did you growl?" Prongs asks frowning, I shrug and turn my attention outside.

There is something about her that just makes me want to be with her, but I'm Sirius Black and I will not be tied to someone.

*James's P.O.V.*

Padfoot got up and said "I'm gonna find Marlene." we nod and he leaves, "I think he fancies her already." Moony says and I nod and add "But he's too proud of himself and his reputation to let it happen. None the less she will be happy with us." Moony nods with a small smile and asks "What will happen if she finds out about..... you know...... my problem?" I look down at the girl that may possibly be my twin sister and shrug "Let's hope that either she doesn't find out or she's cool with it." I say.

The train ride suddenly ends and we walk out.