Ok so I see you don't have a good relationship with your father, but in fairness there's nothing at all wrong with an almost 50 year old marrying a 27 year old.
I don't believe the op said there was anything wrong with the age gap between her father and his wife.
I'd feel weird if my father remarried and didn't tell me and then started a family 27/28 years on from family one. I think it's entirely reasonable for the op would feel unsettled and strange about this news.
When the new baby arrives as others have said it'll be a new little life to enjoy, but don't deny how you're feeling right now- they're valid thoughts and concerns given how your father reacted to your own pregnancy.