it is jealousy of eachother. they need to grow up. have you mentioned anything to your partner about why shes like this? although its so obvious anyway but sure you want to hear it from her mouth what her problem is.
It comes from insecurity. Each of them is insecure about herself and where she fits in. It could relate to where they will fit into your family (wanting the attention to be all on them) or it could simply be that they are insecure about their own looks and think the other is getting more attention or is better looking.
Hard to handle. If you really care about her and want the relationship to last, then you need to just work to make her feel secure in your relationship. (Of course, common decency means you should be doing this with any woman, regardless of whether she is your gf, sister, or friend.)
Women can be very competative with each other, and I do think you hit the nail on the head, that they may be vying for position in the family. Give it some time. You and your brother might want to plan some outings with your fiances so they have a chance to get to know each other. They may come around and become friends. Hopefully they will, because if they don't, they may both put you and your brother in the position of having to choose between them and your family. It's important to a woman to feel like she's the number one priority in her man's life, but these two seem to have gotten into a pattern of one-upsmanship, and that's not good. You can't really put anyone above your wife, so you need to let them know now that it wouldn't be right for them to put you in that position. Yikes. I really don't eny you and your brother. Good luck.