Well, yeah, it's normal for boys to mold his "junk" into most anything it will let them. So glad we can't do that with ours LOL!! And since he has no shame or embarrassment about it yet, it's a given that if he and his sister are naked in front of each other, that he is going to do it in front of her.
However, it's time to separate the "neked butts". If you separate them now, they may not remember that they did it. If you let them wait until they start to realize that they are embarrassed, this stuff will stay with them for the rest of their lives.
No more back to back showers or baths like this. Pretend that they are teens in your mind and separate them accordingly. Don't make a big deal out of it in front of them, but just say they need their privacy, and make privacy the important word about this.
That way he can make a butt of his junk in front of the mirror all by himself! (So funny!)
Dawn