Often taking out a loan against premarital property to make a large purchase of an item that is used equally during the marriage (eg like a lake house they used together) means that the second mortgage amount and the lake house is considered a marital asset. Let’s say for another example that step grandma bought a car and gave it to your stepmom but that your dad regularly drove it, was insured as an operator of the vehicle, and hey maybe even changed the oil a time or two. Strong case to be made that car is a joint asset and the lake house should be the same. Their primary residence is less likely to be considered joint because it entirely predates the marriage and has the step grandmother in the picture. Retirement accounts likely should be split based on what I’ve seen too. Lawyer is needed to untangle all of this and look out for your dad’s best interests, but I will add if an equitable split is offered by the soon to be ex it is rarely worth fighting for 10% more if it drags out with legal fees, so my goal is to get close rather than perfect if that’s even on the table.