Geez Russia.... that is WAY too sensible and WAY too adult.
I used to be a stepmother. The child called me mom and her mother had a fit, so it stopped. Regrettably though, any male BUD that she had was to be called Dad. My children were also very young when both my ex and I remarried and have had more children since. I think the kids just find it weird to hear the younger siblings using Mom and Dad, so we ALL just became Mom and Dad. It's great when the other children try to brag that their parents are still together cause mine both come back with "So? I have 2 mom's and 2 dad's! That's more than you have!"