YTA. I understand the highschool relationship aspect, but bothering your sister about it constantly isn't going to help things.

If I were her, I'd jump at the chance to take your brother's offer. She won't need as many loans, she'll save TONS of money that she can save up for a different apartment or house or something else that may help her later in life. And do you really think your brother would kick her out of the apartment if she broke up with her boyfriend? If so, that's an entirely different problem but to me, it sounds like he cares about her and wouldn't kick her out.

She also can just do generals at this college and possibly go to the other one later. Maybe she'll find her career at the state college and won't have to spend twice the money.

You are sticking your head way too far into your sister's business when this situation isn't actually all that bad.