Rose,
I think that long-distance relationships of any sort are hard to maintain. As human beings we tend to be more concerned with what is happening in our immediate area and to those immediately around us. We also communicate so much in our body language and facial expressions that when those areas of communication are lost we don't feel so close to the person as we did.

Also a lot of our day-to-day chit-chat is about trivial things. If you were there, I'm sure you'd be hearing all about what the neighbours are up to, or how their children stubbed their toes or all sorts of small things. Your sisters are probably thinking "why would she want to know those things from all those miles away?". Your relationship with your cousins has always been different from your relationship with your sisters, so they're used to having a long-distance relationship with you and have kept that up.

Try not to feel rejected by your sisters. I'm sure they still love you but have busy lives of their own and are probably so wrapped up that it's difficult for them to find the time to sit down and pick up the phone.