They’re already taking you and your children away on holiday with them.
She is the mother of their grandchildren, and someone they have known for at least 14 years. She will always be the mother of their grandchildren, and maybe one day grandmother of their great grandchildren.
You? You’re just a girlfriend. In 6 months time, they might not even know you any more. They’re not even choosing her over you though - they’re choosing both of you. They found lovely.
My XMIL has 2 DILs and 2 XDILs, from her 2 sons, 2 marriages a-piece.
The people who bother to spend time alone with her, in order of greatest time spent:
XDIL1
DIL2
XDIL2 (that’s me)
DIL1
Son2
Son1
See how all 4 women including 2 ex wives spend more time than the sons bother doing? I don’t think that’s unusual.
I bet when your boyfriend was married, it was his wife that prompted the social visits, the inclusion in grandchildren’s lives.
You say he can’t even go for lunch because she’s already there? Well how come she got in first? Because she bothered.