Honestly I am baffled by some of the responses on this thread.
OP - you are doing a lovely and thoughtful thing for your nephew. As someone whose family has a variety of dietary needs at Christmas, we all accept that there will be some things we can eat and some things we can't. My daughter, husband and mum are gluten intolerant (quite nasty tummy reactions if they eat it but not coeliac). My sister and husband are pescatarian through choice. My dad has had to stop eating meat because it was giving him heartburn. My autistic son will not eat a roast dinner.
We have a combination of foods so that everyone has something they enjoy.
Last year we did smoked salmon starter for those that liked it, the kids had nibbly bits and played. Then we had a dressed salmon as a main, with a side of ham, and then all of the trimmings of a normal roast dinner, including yorkshire puds because the kids wanted them, along with pigs in blankets. We did two sorts of yp - gluten free and normal, two trays of roasties - one with goose fat and one with olive oil, and my son had a pizza.
Lots of the family don't like Christmas pud, but my husband loves them so my mum made a gluten free one that we all ate if we wanted it, and then we did a gluten free trifle.
It was a huge amount of work, but everyone contributed and helped. That, to me is a family Christmas. If we all had to eat exactly the same it would just be fish and veg and my son would starve.
OP - your brother and SIL are idiots. I would be tempted to write back and say
I didn't realise you'd all stopped eating dairy in support of Adam. Wow, that's really impressive to have a completely dairy free home, well done. I know I couldn't give up cheese without a fight!
I'm sure you're aware, but just wanted to mention that there is dairy in most cake. pizza and doritos.
So, as per Amy and Peter's request, I will make the whole day dairy free. Amy - I have had a quick look and M&S don't do a prepared turkey without butter, so please could you bring one that you're happy with, thanks.
Tescos do prepared vegan mash and roasties, so I'll get those. I won't bother with cauliflower cheese and obviously no black forest gateaux. Mum and Dad - I know you love that, as do I, so I'll get it for us to have when Amy and Peter have gone home. Just to be clear, that's no cheese and biscuits, no brandy butter, and no Irish coffees. I will get some dairy free chocolate, but there won't be any quality street/thorntons/hotel chocolat etc. Amy/Peter - please can you bring a dairy free christmas pudding and dairy free custard or rum sauce that you're happy for us to serve. Thanks
I imagine when it's spelt out to them like that they will change their minds fairly quickly.
Or I'd ask your mum to decide who we're going to keep happy - dad with his cauli cheese and BFG or Amy/Peter with their insane demands. Or maybe ask Adam himself what he'd like for Christmas lunch and just buy that in.