No they don't. Because they have not spent their formative years around their in-laws.
But we are talking about kids here and their experiences of grandparents who they see occasionally during their formative years. That's the fundamental and crucial difference.
If you have a son whose family is blended and he is treating them as a unit (particularly if there is no contact with the other side of the kids family) then you should take the lead on that. If he always treats them as blood, then the grandparents need to respect and support that as a parenting decision and act accordingly. Otherwise they are being petty minded and putting their beliefs and wants ahead of the needs and best interests of the kids.
The fact the son is effectively supporting his wife in how she feels on this, says he feels this way.
In other blended families where the set up is different the situation may be different. Particularly if the other side of the family are actively involved. Step families where this is the case and care of the step child isn't the responsibility of the new partner but the two biological parents have a different set up.
This matters.
The point should be considering what the best interests of the kids are. Grandparents don't get to pick and choose how many grandchildren they have. This includes whether they are adopted, biological or step.
Problems start with favouritism by grandparents. And that happens within biological families too.
If you start treating any grandkids differently you need to be mindful of it and carefully balance that. There may be reasons for treating kids differently - distance being one or the need for one family to have more support. The issue is when you have favouritism which differs from treating differently.
Remember this is about the perceptions of a child who doesn't have full comprehension of adult ideas. You have to be mindful of them possibly feeling rejected, particularly if the split in the family potentially has already had this effect with a parent or grandparent. They need the reassurance of a stable family that they are part of and feel loved by.
So there is no right way to grandparent in a blended family - because the situations differ. But grandparents should support the way in which their child has set up their family and family unit. Otherwise you undermine the security and stability that the parents have tried to establish. If they are doing certain things it may well be because there has been a problem in the past which they have recognised. They are acting in a way they see as being in the best interests of their children and family unit.
I don't think it's wrong for the SIL to have taken child care when she needed it. It was in the best interests of all her children. She didn't need it for the other child. He wasn't being treated noticeably differently. It was supporting the family as a whole.
I don't think it compels grandparents to take all the kids at the same time. The SIL is wrong on this front. It would be fine to have time with the granddaughter without siblings. But in that scenario the grandparents should also try and facilitate time with the step grandkids too. Because it's about formative years, feeling loved and wanted and not rejected as 'lesser'. It's about being a role model for that child. It's about supporting your child's key parenting decisions. It's about giving security and stability. None of that requires you to be bloody relatives.
It's about understanding what matters and doesn't matter in each situation.
And that's why the grandparents are wrong in this on. I don't think it's blackmail on the part of the SIL. She's making a judgement call about the best interests of both children - not just the step son. She doesn't want either child to develop ideas of lesser or better than the other. She wants both to feel secure. If one feels rejected that potentially has implications about the relationship between the siblings by driving a wedge. She's not depriving her daughter of her grandparents. Grandparents are a bonus not a necessity. Grandparents have to compliment and add to the security and well being of children or they are problematic regardless of the family step up. That's why many blood relationships between grandchildren and grandparents break down because of an unhealthy and damaging dynamic which parents feel is not in the best interests of their family unit. It's not selfish it's self protective. A lot of posters are passing judgement without drawing this parallel with blood families which I think is the only relevant parallel.
So yes while I agree there no right way to grandparent, I do think there's a wrong way to grandparent. And that applies whether they are bloody or not or whether they are blended or not.
These grandparents have got it wrong. They are focused on themselves and not on the kids. And that's usually where it goes wrong in any grandparenting situation.