I can do the odd lunch with my mother but a family lunch means DH is invited.
Yes, I think this is the key. Of course it would be odd to resent the idea of your partner seeing members of his family without you there. But if my PIL organized a family lunch, DH would naturally expect that to include his closest family members, his wife & (leaving aside the issue of adults-only events) his children. Fortunately my PIL assume that too.
To exclude me & my BIL would just seem weird, as if they were trying to recreate the past, when they were a nuclear family, & ignore the fact that their children are now adults with families of their own. And when GC are involved, what then? They're "blood relations" so this hypothetical family lunch would just involve the DIL/SIL being excluded, would it? If so, I'm genuinely surprised that that seems okay to anyone.
And that isn't directed at FrauHelga - everyone seems perfectly happy in her scenario, & in any case I do think the question of when GFs & BFs become recognized as partners & start to integrate with each other's families is usually a gradual process. It's also often connected with the parents' relationship with their own child evolving into a more adult-to-adult one, which can take a long time!
Clearly it would be unrealistic for parents to have to start treating every BF & GF as their child's life partner & to treat them equally with their own child, especially when they're still very much in parent mode & the BF/GF has a similar relationship with his/her own parents.
But somewhere down the line, certainly after marriage & children, I agree with bookbag that you'd expect to be seen as part of the family. Well actually, as your own family unit, that's also part of / connected to two wider families. And yes, when ILs don't feel able to welcome their child's partner on those terms (it's about behaviour, not feelings) I don't think they can suddenly expect to be "inside the circle" when it comes to GC, who wouldn't exist without their SIL/DIL.