Frankly, even if we didn't and if stepmom wasn't my best friend? I still don't get this angle. Who cares if a step pops a child on the fanny? Really? Unless that person is beating the snot out of the child, of course, that would be different. It's just one more way of nitpicking at the other parent. I expect my children to respect their father and stepmother even if I don't agree with any of their rules (which off of the top of my head, can't think of anything major), and if they can't abide by the rules, then they suffer the consequences of that action.
You shoulda seen the kids tattling the first time stepmom put them in the corner! LOL! They acted like it was the end of the world and I was supposed to do something about it. My children have learned early and quick that I will NOT interfere and until they show up broken and bloody. Too bad for them.