When my little one was tiny I was just bitchy and instructed people to wash when they came into the house and specified to use the soap next to the sink and sing the happy birthday song.
My husbands grandma doesn't understand but she complies. I can't blame her. Germ theory didn't exist until she was long out of school!
If you really are grossed out by this girl do a family only visiting policy and tell your brother specifically NO