Diaper changing

So I have a 8 almost 9 year old daughter and  a 2 week old son. My daughter has been so excited about being a big sister and has been super helpful. She likes to help change his diaper. I let her do it by her self after I showed her how I also have her do it right next to me so I can talk her through it and help her if she needs to. Today I was helping her change a poopie diaper and my SIL said that it wasn't appropriate for her to be wiping his peinus and balls. I don't think it's inappropriate I mean it's not like she is doing anything sexual she is changing his diaper!! Do you think I'm right or my SIL is right?