I wonder if he actually needs to have the sex himself? Is it possible if the girls go yuri or does he need to actually be involved or can he just come in at the last moment when she cums? I think their thinking on this way too black and white.
Does it even require sex? He could just have a ghost follow someone and wait for them to masturbate. Before that do abilities need to be triggered consciously? Can they be primed to activate? If not then he could stop his sister just by waiting for her to sleep and then make the whole house explode already. Even without abilities, you could just load her up with sedatives like a date raper.