RSVP yes. Call the cousin immediately and ask about the food plans with regards to your medically necessary dietary restrictions, as at this point, it's entirely likely that things are not finalized with the caterer anyway, so it would be easiest to accommodate your medical needs with the greatest advance notice. Then, if you do end up being unable to attend due to having a flare up, it's also in the back of cousin's mind, "Oh, TealDragon mentioned medical problems when they responded to the invitation."