I’d assume if you pressed charges you did call the police and the charge would be assault with a deadly weapon. These are serious charges.

She’s out of control. Why is she dumping her kids on others constantly? I’m concerned for them. If they are taken away from her, where will her kids be placed? This is a very bad situation for all of you, and I am sorry you are experiencing it.

You asked about sibling rivalry. There’s a saying about how the middle child has these insecure feelings and sometimes has more difficulties than the oldest and youngest, for just the reasons you said. Also, your mother marrying your father would definitely be upsetting and cause resentment to her, IMHO. Children can’t help but feel something there when that happens, but maybe sometimes it’s okay for them, not in your sister’s case.

She is dangerous to you, her children, herself, and the rest of the family. She needs help, but what can you do? I’d have reported it to the police, too, since she cut you with a weapon. That’s serious assault and could escalate.