How to deal with discovering a family member's abuse?

I have a pretty good relationship with my parent, I see them a couple times a year since my parents divorced when I was 14 (I'm 33 now). My parent has issues but they have always been very loving, doting and, supportive of me. I found out as an adult that my parent abused my sister (their step daughter) when I was a kid. I'm not ready or comfortable in confronting the parent on something that happened 20 years. But I have no idea how handle this I formation or how to reconcile it! How do people people come to terms with something so awful if you have been in a similar situation?!