The biological parent/child relationship is clear. Everyone knows their roles. Everyone knows the expectations. Maybe not everybody fulfills those roles and expectations all of the time, but even then, it’s usually somewhat clear who did what wrong. There are centuries of built-in guidelines for that relationship. That’s just not so for the stepparent one
Unfortunately, my sister and I have to protect ourselves by unbundling our finances from our dad’s. He was generous enough, when we were younger, to add us as authorized users on two of his credit cards, should we ever need them in an emergency. But, she is also on those cards, and is running up a big tab that could damage our credit scores. So, we’ve asked him to remove us from said cards.