Surely this is a question LW could just ask her son and the fiancée with whom she has a good relationship.

"I know you're looking at venues. It's hard to guess how many people you'll have without actually creating a tentative guest list. Would you like me to send you a list of family members so you can start thinking about whom to invite?"

Then organize the list into objectively named groups, e.g. cousins, instead of subjective tiers.