I go againt popular opinion and am very, very cautious about the 'recovery' of repressed memories. I don't necessarily believe in the term ' false memories' (there is NO such thing as 'false memory syndrome' - it is merely a phrase coined by opponents of recovered memory therapy and is not a scientifically validated 'syndrome' at all), however I do strongly believe that memories recovered through therapy processes are often grossly distorted. The research about recovered memories suggests they can be contaminated by a wide variety of factors, such as fears, unconscious suggestions and expectations, and also that they can be 'pooled' - which means what is recovered as a single memory can actually be a conglomeration of several events merged into one. I do not mean to say that recovered memories are false, because many of them do draw from very real experiences, but the research suggests that they draw from many other sources as well.
Another thing about memory is that we unconsiously change and develop memories every time we think about them. Every time you think about this incident with your sister you are unconsciously developing it further - when you think that it might have something to do with sexual abuse you are unconsciously creating it to be that way. It may be that it was a true sexually abusive event, or it may not be. The nature of memory means that you may never know for sure one way or the other, although many supporters of recovered memory therapy will argue otherwise.
I am certainly not saying that all memories are false, or that what you are remembering is not true. I caution you against 'searching' for more, though, because anything that is remembered from here on in will most likely be contaminated by belief, expectation, suggestion and other experiences you have had.
I know my opinion will not be popular, but it is what it is.
As for a way to move forward... what I learned when trying to find my own way through the confusing tangle of recovered memories was to trust and heal the emotions that came up. It didn't matter what the source of the feelings was, it was them that needed healing. In my own therapy we worked through the memories with what I call 'suspended belief' - we dealt with what came up, but without necessarily labelling the memories as actual truth. It might sound weird, but it worked. I guess I took the stance that my mind was creating 'stories' to enable me to work through the trauma. I knew there was some truth to the memories (some of my perps had confirmed abuse had occurred), but also knew there was distortion involved too. Ultimately I don't think absolute truth is important - what is more important is to work through the feelings that keep causing you trouble, regardless of their source.