What do I do? My father died in 2007. My sister confided in our other sister that our father molested her for 10 years. For many reasons, too many to explain, I'm not sure if it is true. Do I tell my mother who is 92 years old, or keep it a secret?
Only your sister had the right to tell someone that. It's hers to decide in whom she confides. Do not take her agency away.
It would probably do no good to tell your mother anyway. Either she already knows, knew at the time but didn't want to do anything, or you are shattering her idea of the father of her children. let her be.
Offer your sister support. Maybe she is lying. But always work on the assumption she isn't.