I'll add that I have an adopted child and another child who is mine biologically but not my partner's (because we are both women, but the child was conceived within our relationship). I am fully aware that individuals within our extended families feel differently about the two. For me, how they 'should' feel is a moot point - I can't control that. But I expect people to have manners and if they have a relationship with my child I expect them not to act hurtfully. So, I have aunts and uncles who made a big fuss when I had my birth child, but completely ignored the arrival of my adopted child (and ignore her to this day). Because I very rarely see them, I don't see any need to try to resolve this.
But the grandparents and my own siblings, I do expect more of because they have a relationship with my child. I know for a fact that my mum feels more love for my birth child, but she doesn't show that to the children and I know I can trust her to protect dd2's feelings. Their other grandmother has been more tricky, and for a couple of years I stopped seeing her because - while wanting to be called Granny - she was mean and excluding to my dd (asking me to leave the house when 'real family' came visiting, FGS) and treated her very differently to her biological grandchildren. Of course she has the right to her feelings, but I had the right to withdraw my child from exposure to that. Now that she is kind to my children again, we are back in contact.
I fully expect that when she dies her money will be left to her biological grandchildren, not mine, and that is up to her (though a shame because her other grandchildren are already very wealthy and mine are definitely not!).
Our children don't ask to be put in blended families and I don't think it's too much to ask all the adults involved to be a bit aware and caring about their feelings. In OP's case, the grandparents could have either included her son (because live children's feelings trump all), or made sure he never saw the notice, or agreed with the parents a way of explaining the rationale, e.g. "You know you're not biologically our grandson, but you are 100% ours in the most important way - in our hearts, and because we couldn't love you any more. But x was not your great-grandfather because he wasn't related to you and he didn't know you either, so it's a bit different. When I'm dead and gone and I need a memorial written, I think it should be your job to do it and I expect to see your name in big letters - maybe even a photo!"" or something like that. My personal choice, though, would be for the grandparents to just keep the memorial as their private thing, and not involve anyone in OP's family.