Rehearsal Dinner Invitations

My mother is having a rehearsal dinner for my brother`s wedding. Should only the actual wedding party be invited to the dinner, or should their spouses also be invited. My mother has said she has only invited, and plans to pay for, the wedding party`s dinners. What happens if spouses and kids also attend? Is it proper etiquette to tell everyone ahead of time that only the wedding party`s dinners are paid for and anyone else that attends should pay for their own?