It's not incest if they are step siblings and this is kinda nice.

Turning into lannisters I guess.

'When you marry the king, Joffrey's mother will become his sister-in-law. And your son will be Loras'... nephew? Grandson? I'm not sure. But your brother will become your father-in-law, that much is beyond dispute. ' Lady Olenna Tyrell

The only way I would understand her being mad is if her parents were still married and this started as cheating. Otherwise it's not weird it's not like once they marry op and his wife become blood siblings. Let them be happy and be happy for them

This reminds me of a weird and awkward situation. My step sister's mom dated our dad's (my step dad's) father. My whole family was creeped out by it, and incredibly angry as well lol. They broke it off not long after. Good luck to you, op. Hopefully your dad doesn't go through with it. o.O

"Pst did You know my dad is banging Your mom?" Relax and hope they use protection or You'll have uncomfortable situation with siblings.

Awkward yes but if two people are happy no matter who they may be more power to them

Very interesting family dynamic...

...And your kids will be your cousins!

It pretty odd, buts it's not incest or anything. You should be happy that your dad found love, even if it's your mother in law.