If you have no problems with it, don't worry what others think. Other people don't know your life or your step son like you do. I used to bathe with my brothers who are 7 and 8 yrs older than me, up until I was about 3 or so and only stopped cause they wanted their privacy. Children grow up all too quickly nowadays. It's sweet that he wants to be part of his sisters life and help out. I would let him, he's only a little boy! As you say if he was a she, there's be no problem. And I'm sure he wouldn't be left alone with a tiny baby anyhow, you'd be thereall the time, so no worries. He's gonna grow up soon anyway and will want privacy.