in general step children often do get treated very differently from how someone would treat their own children.
Of course they are not treated the same way.
Your stepchildren are your in-laws. If we are talking about adults (and not even young adults here), then a stepdaughter treating you with contempt is no different to your sister in law or your mother in law doing the same. You aren’t going to feel as you do about your own children (although no one should be letting their adult children dictate their lives and relationships or treat their partner disrespectfully).
Why should anyone put up with that? As a stepparent, they don’t (and probably never did) have the authority to challenge poor behaviour in their stepchildren. And now they’re dealing with an adult who may well continue to treat them poorly simply on the basis that she’s their stepmother. It’s not uncommon.
After all, the stepchild is entitled to feel however they want about their in-law relations. They’re just not entitled to act like arseholes.
Obviously her father will feel differently. But he should be tackling her behaviour. As a parent it’s completely reasonable to say to your 30 year old daughter that she is not welcome in his home if she won’t treat everyone in the household with respect.
Part of being a grown up is dealing with the consequences of your actions. If you are awful to people, then it will limit where you are welcome. Even within your own family. Being related to someone doesn’t mean you can be an arsehole to them (sadly far too many people seem to think it does).