two boys...NEED HELP!

ok so i am babysitting my friends two kittens. they are about 8 months old and they are brothers. well one of the boys constantly sucks on his brothers manhood. they are not neutered yet, but will be in a couple of weeks. now is this normal for a kitten to be doing this to his brother? and what can i do to stop him? please help this is getting really annoying!

Kittens not properly weaned tend to suck on anything. Hopefully he will stop on his own but his brother once annoyed may be the one to "kick" him off the habit.