Completely baffled by this whole post & thread!! As a few pp have mentioned, why would anyone's family not
a) ensure enough seating?
b) check whether you were comfortable or wanted a seat during a whole day sat on the floor (at Christmas too)?
As much as OP could have spoken up / got a dining chair, so could any of those other adults in the room, yet they didn't (very odd)...to my mind this could indicate a family that lives by diff values than my own eg there are parents who think nothing of putting a child in a cage or feeding them from a dog bowl on floor...that's considered abuse of course & perhaps OP didn't speak up as their family are emotionally abusive so think its fine they sit on floor all day - for future ref its not.
If the problem is they just weren't being considerate when a higher number of people / visitors present, its still weird...I haven't been at a large family gathering for a while (the last was Aug parents 50th anniersary party at my sisters house, some chairs indoors, some in garden, people played musical chairs, etc. no issues...we never have large Xmas gatherings now - sister in Leeds, another in Thornbury, we're in Wales, my folks in Sidmouth, ILs in Windsor - so this isn't an issue, but any event like this people would move heaven & earth to offer guests a dining chair, etc. so I am completely fascinated by OPs family's behaviour...elder relatives would rarely sit on a floor, younger ones might, but would be offered a seat at least; even my dad, 71, sits on floor to give out presents, my ILs at 73, less so).
OP may have a number of reasons for being shy to speak up: ASD, is a long-suffering recipient of this sort of behaviour from family so didn't feel able, was the only teenager present, so didn't feel able, but presumably is a parent so deserves a comfty chair as much as any of us!
Failing all this OP, next time, bring in a dining chair or sit on your rude sister til she gives up her seat; if you are Monica from Friends size that will be a hugely effective tactic going forward.