Frozen shrimp don't work for a boil; if they are not fresh they turn into mealy goo. Absolutely not worth bothering with. (Unlike crawfish or crabs, shrimp are not boiled live, but you need to keep them fully in the shell and with heads on to cook them properly by this method.)
1.5 lbs per person is the standard portion for a boil, but that measurement is for head-on larges; the size that non-coastal folks think of as about 20 shrimp per lb. raw weight. In practice, most people err on the side of generosity and go for a ratio of 2 lbs pp., since at a boil you usually sit around talking and munching for several hours. I just checked, and local market price for these in Coastal Louisiana was running about $8/lb before the storm; you mostly cannot get them just right now because the dock facilities were hit pretty badly, and a lot of boats are in for repair.
OP, what I would do is call your SIL and ask what the expected per-family contribution is; if yours is abnormally high, then you're really a co-host, and thus get a say in the party planning. Who is covering the price of the other boil components? Stand firm on not spending money on t-shirts since the price of the food is up so much this year; & if necessary, counter with the option of a smaller souvenir instead, like a beer koozie or a water bottle cover, with a limit of 2 per family, plus stickers the kids can put on their shirts if they want to.
(I'm on the committee for a big annual non-coastal boil for a charity. We thankfully just charge all the guests a plate fee, which varies depending on the market price of the shrimp. We normally order 3/4 of a ton for the party, so we get a good price, LOL.)