I think how much or if at all this affects you as a child must differ from person to person.
I'm a stepchild, I have never had much involvement at all with his parents. Never even met his dad and he's been married to my mum for 12 years. I've met his mum but she's not my grandma, I have those already.
It has never ever occurred to me that I should be hurt or damaged by the fact some woman I barely know and who isn't my grandma, doesn't treat me like her granddaughter.
I love my stepdad to pieces, he's not my dad, I have a great one of those already, but he's a valued and brilliant family member of mine who has always treated me well. Tbh the relationship with him is the only one I'm arsed about.
I must say I don't really get why someone would be damaged that their step parents family didn't treat them like a grandchild. Especially if they already have grandparents anyway. I don't see what it is they think they should be receiving from these people who aren't actually related to them or what they think they have lost by not doing so.
And the argument always falls apart for me when people go on about siblings needing to experience the same things I.e. the disney trip here. The stepchildren aren't necessarily received "less than" the shared DC because their grandparents are taking them somewhere, in the same way no one would suggest the shared DC are being treated "less than" if the DSC were taken somewhere by their maternal grandparents.
They are siblings yes, but they happen to have different families so will sometimes receive different opportunities, and that goes both ways, DSC will receive things from their mums side that shared DC will not. It's just life in a blended family surely?
I don't get why people always try and mash a blended family into a seamless nuclear mould. It seems a bit daft to me.