Sadly, because of dynamics involved it is quite common that the abuser gets away with his / her behavior. And sadly those who have suffered at the hands of an abuser are at a higher risk to become abusers themselves.
If you have not experienced such a thing yourself, it is quite hard to believe your brother, sister, aunt or uncle would do such a thing. And even if you "see" some signs, you may end up interpreting them in such a way that you don't see them.
And another thing, without conclusive proof not much can be done. We had a case like that in the Netherlands, in which supposedly a police detective was involved with his daughter. His whole life was ruined, and did not change when it came out the daughter made the whole thing up.
It is hardly desireable that innocent people suffer from false accusations, or that guilty people make millions in "damages" when there is lack of evidence.
All this does not help victims of abuse. Society needs to find effective ways to help and protect victims.