Disagree entirely.
If this was the OP and she said that his parents didn’t so much as acknowledge her, people wouldn’t say “you can’t expect his parents to treat you as one of the family,” except that by being with him she then is part of the family. And by extension, if you get together with someone who has children they are part of the family.
No maybe they’re not grandchildren in the same way, just as the OP isn’t their child, but they are part of the family because they are the family he has chosen to be a part of.
There is far too much allowance on here of treating step children as second class. If they’re not included then what should they expect? Why should they get presents, their mother/father’s family should give them presents.
It seems clear that may people just view step children as an inconvenient part of their lives, a part which has to be there and tolerated if they want to be with the person they’re with.
The OP’s do should stand up to his parents. Personally I wouldn’t stand for it, and if my parents so blatantly ignored children I had chosen to make a part of my family, they wouldn’t be seeing my children either.
The more and more I read on here about the attitudes to step children, by family, by friends, by random MN’ers who seem to think that step children shouldn’t expect to be considered, the more I think that this whole idea of blending families needs to be outlawed. Because too many children are made victims of situations they never chose to be in in the first place.