It’s also possible to love your children but not like them or their behaviour. You can say to an adult child that you aren’t willing to be treated poorly (or have them treat other members of your household poorly) and that you aren’t willing to have them in the house if they aren’t willing to behave in a civil manner.
The OP says that the 30 year old daughter is making it a ‘her or me’ choice for her father.
The OP is entitled to have her home be a place where she is not abused and treated with contempt. No matter who is doing the abusing. Things are more difficult when it’s a child who comes into your home and treats you badly, but when it’s a 30 year old woman, it’s absolutely fine to say that you aren’t willing to have that in your house.
And (as is so often the case in stepfamilies), it does fall to the parent to put boundaries in place and have behavioural expectations for their child(ren). They cannot expect their partner to simply accept their children’s awful behaviour on the basis that ‘they’re my children. They’ll always come before you’. If they aren’t willing to do so, they deserve to be single and to deal with their adult children’s unpleasant behaviour all on their own.
If I thought it was ok to turn up at my mum’s house and treat my stepdad like shit, I’d expect to be told to leave. If I did it consistently, obviously I’d no longer be welcome. The onus would be on me to improve my behaviour if I wanted to be welcome there.
Just because you are related to someone, that doesn’t mean you are entitled to treat them poorly. Nor does it mean they should accept you doing so.
The difference in stepfamilies is often that only one of the people whose home it is is able to do anything about the children’s behaviour. Just as when your MIL is being a dick, it’s often difficult because only the person with the blood relationship can actually sort it out. You’re screwed if he doesn’t do anything on the basis that ‘she’s my mother/daughter/sister’. If it’s your shared child, you can tell them off and put the boundaries in place yourself. If it’s your stepchild, you are reliant on your partner doing so for you.
That is why people don’t respond to SC like their own children. Because they aren’t.
If your own children are behaving dreadfully, you shouldn’t just let them ‘because they’re my children’ either.