Inviting a 15 year old to a sleepover of 10 year olds

My twins (boy/girl) twins are turning 10 and they are having a sleepover. My daughter can invite 4 friends and my son can invite 4 friends. Both my son and daughter wanted to invite this brother and sister. They both have autism. The brother is 12 but the sister is 15. The sister does hang out with my kids. Neither her or the brother have ever been to a sleepover. Their mom told me they both still need pull ups at night and have trouble making friends. The brother is closer in age to my kids but the sister is quite a bit older. I don't want her to feel left out but would it be inappropriate to have her here?