Do sisters need to be involved in choosing the right girlf for his brother.

I don't think you should. I hated that when my brother tried to get involved with my boyfriend decision. For one, what's good for you may not be the best for the other. We don't have the same preferences and likes so we can't really say that what we want is the best for our sibling. I guess the best thing you can do is modeling. You can model a good personality for him so he can try to look for someone like you. As for my case, I hate my brother's personality so I looked for the exact opposite. Actually, I was looking for someone like my dad, who was my role model. I think that works better than to really put your foot on the whole thing.

As long as he's happy and loves her, we'll be happy for him. I don't think it's really the sister's place to "pick" the girlfriends but some helpful advice/opinion can never go astray!

Unless there is something major and obvious about why you don't like her, like she's cheating on him, let him make his own decisions. If he is not mature enough to understand, then he's probably not mature enough to date. Give him some credit. Let him choose for himself who he wants to be with. If he makes the wrong choice, that's ok, we all make mistakes sometimes. He needs to live his own life. If he comes to you and asks, give him your opinion, but still let him be the one to make the decision.