"I think I am allergic to alcohol" worked for me, followed usually by some sort of joke
whenever I drink I break out in handcuffs
the last time I drank I woke up with a Sasquatch so I'm taking a little break
or something along those lines
It soon gets easier, I just learned to set a boundary and keep it, not try to explain it or defend it, that made it worse, just "I'm sorry, I can't join you tonight" I found if I tried to explain my alcoholism to people I drank with they got incredibly threatened and then we'd argue, so I learned to just be low key about it until I learned how to navigate these situations a little more skillfully.
Also, something of CRITICAL importance, is bring my own ride/car in order to make an escape if necessary, I found family affairs to be nearly intolerable in early sobriety