I think the mere fact that you're the younger one would lead to a bit of jealousy for the elder sibling. That and the fact that you both needed extra care is probably just a recipe for a giant pot of resentment. Although I'd think that jealousy would fade as your sisters get older, since they should no longer have that childish "I need all the attention, how dare she take it away from me!" mentality. But I guess a lot of the time these feelings we have in our childhood years carry through.
In my case, I think the main reason I didn't have that jealousy is because of the fact that I really considered my sister as a present to me. I only had brothers, who were all older, and growing distant from me, as they moved further into the typical activities that males participate in. I really, really wanted a sister. So, like I said, she was like a gift to me. I find myself waiting for her to get older, so that we can be even closer. I kind of feel like a bit of a weirdo now, haha.
But I hope both your sisters figure out that what they're doing is silly. Life's too short to waste resenting your own blood for absolutely nothing.
Attitudes are contagious, make yours worth catching.