That sounds like you should be fine.
Figure #26 brisket will cook down to aprox #18, most people won't really eat more than 1/5# of meat with sides. Plus throw in the ribs at about 3 ribs a person with the brisket and fixins.
You should be golden.
We used to do an end of summer picnic at the farm and had 100+ people, we did a rear 1/4 of beef, aprox #115, before cooking and had left overs to freeze.