Dysfunctional families will often isolate the injured person (you) in cases like this b/c to do otherwise would mean that _they_ would need to own up to their own issues or part in what happened to you.
I'm sorry this happened to you. But you are beating your head on a brick wall if you expect that your family will come to your aid. You *should* be able to expect it, but they simply can't for the reasons I said. You absolutely need to get yourself a support network & counselling.
Have you tried calling your local women's shelter? It may sound far fetched, but they will have rape resources you can be referred to. The amount of time passed doesn't matter. There are lots of women (unfortunately) who didn't even realize until years later that what they experienced as a child was actually rape. And since this has obviously deeply affected you (as it should) I think you should avail yourself of this help immediately.
Your family sucks, btw. I feel for you. But you have done all you can under the circumstances. You told them what happened, which your sister needed to know both for herself & her child. If she chooses to stick her head in the sand that is her path to walk.
Meantime, get yourself healthy. You may have to intervene on behalf of your niece sometime in the future & you want to be strong enough to do that. Keep your chin up & don't let other ppls issues change what you know to be right.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh