Peter TBF I didn't read the entire thread so I don't know if his behaviour changed and he got crazy.
I agreed with the fact he was entitled not to raise /finally support another person in their house.
IMO a lot of posters were infantilising the SD. She's not 16, she's 18. An adult. And if she chooses to bring another life into this world then she needs to step up and shoulder the responsibility, including putting a roof over its head, not expecting others to do it.
The H already made his feelings clear on young pregnancy so they should have known how'd he'd feel. It was hardly a shock. You don't always agree with how family feel, think, behave, but there are ways to deal with it, instead of breaking up.
Do you never forgive the racist gran? The homophobic uncle? If you only accept your family by your standards, you wont have much family left.
As it happens, a similar thing happened in my family. My aunt who's younger than me got pregnant at 19. She knew her parents would not support her having the child so without telling anyone she was pregnant she left. Moved to an affordable (for her) place. Got herself sorted, told everyone. It was a struggle but she avoided the conflict and her parents see her and her child often and adore him. By MNs opinion it would make them terrible people she should have cut out from her lives. If she had her and her son wouldn't have the loving relationship with them they do now.
But as I said, OP knows her situation best, we only know fragments. It's up to her how she goes on.