It is normal curiousity.Don't make them feel ashamed or embarrased. Just keep an eye on them both or separated. It is normal for kids to touch and feel and compare. Adults just have to control it.
Who was supervising them at the 5yr old Uncles house? Some curiousity is normal for Children of this age however make sure it is not being copied from an older child,, ask her why she does it and get some answers, play it down but me more observant.