Technically I'm a step, but with BM out of the picture and the kids only being 5 and 7, I am mom.
That being said,
Stepparent asked him if he would enjoy being forced into a shower with the water on, he indicated yes. I (Mom) explained that was talking back. I subsequently asked the same question again moments later, and he again said yes.
I'm very confused as to how answering the question asked was talking back. I feel we're missing information here.
Stepparent got up out of nowhere without asking me and grabbed the 9 year old and started to take him into the bathroom. I told stepparent to stop multiple times
THIS IS NOT OK. By your wording you've indicated that this surprised you and that's alarming. What's more alarming is he ignored you.