It sounds to me like your sister is having a hard time accepting the fact that your little brother will likely have a much better childhood than she had.

Still, yelling and getting emotional doesn't necessarily make someone an asshole. Communicating with others can be difficult for a variety of reasons.

Your mom grounding both of you seems a little bit over the line to me too, but it's a decision she made so you'll have to ask her about it if you want clarity.

For the most part this just sounds like regular family problems that need to be worked out, and because it's going to involve a lot of strong emotions it's probably going to be pretty intense, but it doesn't seem like there are any assholes here at all, just people who want different things who have a hard time communicating their feelings to each other.

NAH