SUBREDDIT: r/relationships
TITLE: I (24F) with my Dad (58F), he has a messed up past that he doesn't know I know that explains the way he acts. I don't know how to be okay with it.
POST: A while ago my Dad got extremely drunk around my little sister (20F) and told her three major things. 1) He was in an orphanage for a period as a child 2) He was married for two years when he met my mum 3) He was sexually abused as a child. 

My sister has slowly told me these things as she's frustrated at the position that he put her in by telling her those things (and not remembering) and also to talk about it. 

My Dad has always had anger issues and will become stressed very quickly over stupid stuff - where he starts yelling and turns bright red. For example my sister has lost her bank card overseas so I'm going to do a money transfer for her- he's stressed that she has lost her card, blah blah. I live with my parents currently and they came home whilst I discussing it with her. He wasn't asked for any assistance but felt the need to comment negatively about it. It pisses me off because my sister is doing the right thing and it will be sorted soon. 

He is really insecure about himself but is overweight and constantly making excuses. He is judgemental about so much and quickly dismisses people if they don't fit his idea of how people should act. He has a very dysfunctional relationship with his sisters and when you know about his childhood it makes a lot more sense and I can have empathy for him but I'm not meant to know. 

I wish he would've told us all at some point but I can see why he didn't. I'm struggling most with the marriage thing, that seems like something you'd tell your children doesn't it? We don't have any other brothers/sisters apparently. The marriage is news to me today and I've known about the other points for a while now. 

How do I keep quiet and not start resenting my Dad for keeping these things from me? He had a terrible relationship with my Mums parents and always talks bad about them- but they likely don't approve how my mum and him started their relationship.
TL;DR: 
My Dad has had a bad childhood and was married when he met my mum. He told my sister. She told me and he's not the kind of person you can talk to about this stuff. How do I deal with it?