What can be a sibling relationship that causes problems for the girlfriend?

From experience I have noticed that it could be jealousy, usually(thought it can be the opposite)on the sister’s side. She may see a long-term relationship as a threat.

Hmm…another vote for jealousy, depending upon the siblings’ relationship. When I started dating my roommate’s cousin (they were as close as brother and sister), it changed her attitude towards us. This could be wrong, but I think it had to do with the fact that she now had to ‘share’ him or didn’t have as much one-on-one time with him.

Maybe it is because the sister liked a previous girlfriend of the brother’s better. Or maybe sis doesn’t think the girlfriend is good enough for him. Then there is always the chance that the girlfriend is great and could do better than her brother. Oh, here’s another thought…maybe one or both parents have voiced concerns about the relationship and the sister is only mirroring their sentiments. What about this? The girlfriend has characteristics that the sister doesn’t appreciate or accept, but the brother values.

When I was in college my brother started dating one of my friends. I knew things about her that he didn’t and it bothered me. Maybe there was jealousy too. He was my big brother and I looked up to him. I didn’t see him much as it was and she was taking time away from me.
I would handle the situation much differently today I hope.