How often do you give a 8 week old baby a bath? am i over stepping?

My sister in law is young, and just had her first baby. So far she seems like she is going to be an awesome mom, but i do have a concern. She doesn't have any "parental" guidance and really hasn't been around a baby too much. I watched my nephew on saturday, he had this cute little winnie the pooh onsie on. After work, she picked him up. On monday when i babysat him again he was in the same outfit i thought...ohh she must like this out fit. Tuesday comes, she comes over just to visit, he is in the same outfit my husband said don't embarr her by saying something. Ive seen him in other outfits, before this so i do know she changes him and bathes him, but.... If he comes over today( i babysit again) and he is in the same outfit, I am going to give him a bath. Am i over stepping this? how do i ask her if she needs help? maybe she isn't comfortable giving him a bath? I don't want to look rude