The sizes listed in this post are for 'party' servings.....NOT wedding sizes
Yes, one can add another tier using styro OR make the cake bigger than 100 servings and give out the extra to family/guests to take home OR cut it into say 1/4ths, wrap well in plastic and freeze for later occasions. With summer coming up there will be lots of picnics and other chances to serve any leftover cake.
Since this is for family (and I'm assuming you are gifting it) I would just make her the cake that is the size she'd like to see and not worry about the serving amount (since it will easily be 100). If it was for a paying customer, they might not go for that since they'd have to pay for it.