I'm a mom, not in a mom "role".
I'm not playing like I'm something or someone I'm not, I just am the mom.
Only those who aren't the mom use the word "role" as if there is some sort of opening when mom isn't around on weekends or half the time or even if dad has custody. There isn't an opening, though. Mom is mom at all times.
I doubt SM would allow mom to come into her home to take care of her children when dad won't or can't, so mom isn't not doing her job. Dad is supposed to step up and handle his children and take care of them when they are with him and that is why there is no opening. The parent slot is filled in both houses.
There are no try outs, there are no openings. There is just mom and dad and SM and SF.
If you feel you are playing a "mom" role in your home you have to understand on some level that means you're only pretending.