Yeah, I have to say, I find it weird that she is doing it. I know I wouldn't be - not with my first baby still only 3 weeks old! Stuff the tired issue, I want to be with my baby, not taking someone else's to swimming lessons. I find it odd that she is willing to do it, but more odd that he is going to a swimming lesson at least ONE of his parents can't attend.
What is she going to do with her baby while she is doing the lesson with your son?
Have you ever been a step parent Lime?

Don't want to offend, but your post is a little offputting. The lady is not taking "someone else's kid" she's taking her step-son, who lives with her 50 percent of the time. It's not odd at all that she's willing to do it. And also, the child is 3, she doesn't need to be in the water with him.