My husband left us..should i confront the mistress? Should i inform his family?

In my opinion, I think his mind has been made up for the time being; however, I wouldn't confront the mistress or his family until you know what his intentions are regarding aiding your family. Does he still plan to help out with you and your children? If not, then you have EVERY RIGHT to approach the family and figure out something mutually beneficial for both parties.