My FI's aunt took it upon herself to remind her family to mail the RSVP's back to us. Well, she "reminded" family members know who weren't invited. Now his other aunt wants to know if she can substitute one of these uninvited guests for her husband, who can't make it. FI says, knowing his family, if we allow it in this situation, his family will all be wanting to make substitutions. How can he polietly handle this? We're both at a loss as to how to say no.