toddler boys

I have a one year old boy and my sister has a two year old. We've caught the two year old a few times looking at my one year olds boy parts. He will take the one year olds diaper off and "play"with it. How do we get him to stop?????

It's normal for them to explore
Tell him that you can't touch other people's private parts

its very normal for them to do that just tell him that you cant touch other people like that

We've done that and he just keeps doing it.