Cook it as you would normally do , serve everything up in dishes and meat on a sliced on a platter, let them help themselves, Turkey is low fat so shouldn't be a problem, but the trimmings that qo with it might, and if they help themselves they can just have what they want, a salad type started with prawns or smoked salmon should be low fat too, a fresh fruit salad for dessert. If they over indulgence it's up to them to put up with the consequences. I would offer some plain boiled potatoes as an alternative to roast or mash, if they are not eating they can be made into a potato salad for boxing day. Are they staying with you? If so get extra loo roll in, in case they over indulgence as that is the negative with the arrangements injections.