Telling estranged father

Ok. This may be weird but here goes:

My father left when I was 2(27 years ago)while my mom was pregnant with my sister. In 2015 he suffered some health issues and we reconnected and visited him. Emotions, likeness, etc happened. I am JUST like him. Its insane. We've been talking off and on, like a facebook message here and there, since. Well, as expected, my sister isn't too keen on having a relationship with him and I understand why completely. Everyone in my family thinks I should just let him go too but....he's my dad, I had a father, in person, for a whole 2 days and it was honestly amazing. Fast forward to finding out I'm expecting. Well, I want to tell him and give him a chance to be a grandfather. Mom says Hell no. Make him wait. I don't want to keep this from him. As much as he hasn't been there for my mom or my sister...or even me, I'd like to give him a chance. I don't know the full family drama story and I honestly don't want to. The past is the past to me. What would yall do? I feel like I'm going against my family if I let him be a part of mine and my child's lives but I also feel like something is missing with him not being involved and then me stuck in the middle occasionally responding to messages while on the inside I'm dying to have a relationship. Comments and suggestions are welcome. Anyone else have this insanity?