It would be a good idea for your Mom to take her to the Doctor and ask him then get an evaluation done,it is always good to believ e the child and check out the facts, make sure she is never left alone with the cousin, there are ways a counsellor will ask specific questions and know by the answers ,if you beleive as you have said that it is true you need to protect your sister ,find out what the abuse consisted of,it is possible the Counsellor would know how to do this so not to cause your sister any more Trauma.What do your parents say to this how have they reacted , it seems they should be very concerned.