Totally normal. I am constantly telling my now 4 1/2 year old, to get his hands out of his pants (front and back!) and to wash them afterwards! My 2 year old is slowly toilet training too, and is becoming more and more fascinated with his penis and bottom.

I have taken to telling him he can touch himself when he's in his room or in private, but it's not appropriate to touch it when other people are around (especially me, it's rather disturbing to see my 4 year old with an erection!)

Thankfully I haven't experienced any poking of siblings bits yet!