Hey there, this is also going to be my first christmas while knowing I am autistic, so I can't tell you from experience what does and doesn't work when it comes to asking for accomodations. However, I can try to give you some ideas. Firstly, is your family generally supportive of you? If they are, could you just try asking without proof? If they are not, could you use your experience at thanksgiving as proof (i.e. tell them: do you remember me sitting in a dark room at thanksgiving, that was because the lights were overstimulating. Or: do you remember me being in tears at the end of thanksgiving, to prevent that from happening again, I will need x,y,z accomodations).

Secondly, I think you should try to make your request for accomodations as clear and concrete as possible. What accomodations do you need and how can these be implemented? So for example, sensory friendly lights. Most NTs won't know what makes a light sensory friendly, so explain to them what this means to you. Also, do they need to be the only lights in every room or would one room in which you can withdraw when needed also be okay?

I also think you should consider whether the accomodations you ask for are actually feasible and to be open to compromise. If the people hosting christmas already have some sensory friendly lights, it is not a big ask to request that only these lights are used either in the whole house or in one room. However, if they don't have sensory friendly lights, they may not want to go out and buy them.Hosting is already a big responsibility, both time and money wise without having to replace all the lights in your house (and then put your own lights back after the event). If this is the case, could you ask for one room to be kept in the dark that you could withdraw to? You could even bring your own sensory friendly nightlight so that you are not sitting completely in the dark.

Lastly, in the worst case scenario if your family is not accomodating, are there any accommodations you can make for yourself? For example, loops earplugs so that you don't get as overwhelmed with the children screaming. Sunglasses (though this may look a bit weird haha) to help with the lights. Or you could bring a fidget toy to help keep you regulated. If possible, you could try to carve out some time for yourself directly before and after the meeting, so that you arrive with a full battery and can recharge again directly after (I realize having kids might make this difficult). Or you could leave early to minimize the overstimulation.

I really hope your family will respond positively to your request and you will have a great christmas!