1/4 to 1/3 lb per sandwich is about right. So, estimate the size of your in-laws (yeah, I know...:) ) and go from there per sammie. Remember though that you will only get about 1/2 of your starting weight on raw meat give or take depending on how much fat you "cull" out while pulling.
So, if you have 12 folks, and think that ALL of them will eat 2 sammies, and you have the big buns and they're big people, then you need 12*.33*2= 7.92 lbs of finished meat (that's based on the 1/3 lb per sammie and 2 sammies per person), and that means you should have somewhere around 7.92*2=15.84 lbs of raw butt.
At 1/4 lb per sandwich you get 6 lbs of smoked finished meat (2 sammies each still). That's what I usuually use and looks like the rest of the folks do too that have answered.
It would be different if they weren't your inlaws....;)
You do NOT want to run out. Leftovers will be yours, and will taste 10X better when you get to sit back and eat it without any "judges" around .:p