As long as you are a relative who knew the person in your lifetime (e.g., you can't say "personal knowledge" when you are documenting a person who died in 1850), I don't think most people would question that. So one way to state it would be "personal knowledge of Emily Metzger, daughter," for example. In the long run it will be helpful, as you appear to know already, to have more documentation--usually, the more the better.