Firstly, I am not advocating for step children to be treated badly in anyway.
But I think it is fine if grandparents do not give gifts or gifts to the same value to the stepchildren as to their grandchildren. If there was a divorce the stepchildren would generally not be seen anymore anyway.
People generally would not expect grandparents to give their non related stepchildren an inheritance. Those who advocate for stepchildren to be treated exactly equally, do you think they should inherit from non related grandparents?
I am talking about stepchildren in this instance where ones DD or DS has married a partner who has children from an ex partner.
Same for family holidays. Often grandparents will pay for a family holiday and want their grandchildren to join them. They should not have to pay for the step grandchildren also.
Stepchildren can end up with four sets of grandparents.