Are your from India?
Ignore the one that said you need to see a therapist. We on this board were raised more the way your sister wants to live, so you aren't going to get people who really understand you here.
Your sister fell in love. Be supportive of her decision or you might lose her. Meet the boyfriend, make sure he's a good guy. Encourage her to finish her degree, no matter what it is in. If the boyfriend disappears, she will still have her degree.