OP, this may not be your situation, but I thought I would give an explanation of my situation to illustrate that there can be good reasons for a Grandparent to want some control of their money.
My son, our only child, is four. We planned to start a 529 for him before he was born, and opened it within a month of his birth. We thought it was important to share this with our family, to communicate that education is important to us, and to invite them to this shared project.
When I talked to my in-laws about it, my FIL had a very astute question. He asked, "what will you do if he chooses not to go to college?" We had definitely thought about this. My son's only cousins are kids of my brother. They are a great, loving family, but they are not currently saving for college, so will likely struggle to fund it or have to take out loans. So I had a ready answer that we would name the cousins as beneficiary.
My FIL is a great guy, but he pointed out a problem with that--he wasn't putting his money for someone else to go to college; he was concerned about his heirs, including my son. That thought really let the air out of my balloon, because he was totally right. We agreed the all-together 529 wasn't going to work. But FIL still wanted to help some way, so went back to think on it.
What they did is they put a gift to our son in their will, with an amount payable when he's 25. They know that we will have a 4 year, public school degree covered. They have shared the amount they have set, which would cover 4 years in a private school, frankly. But if all works out well, then he has a house, or a good down payment on a house. Or grad school. A leg up, for sure,
It's an amazing gift, and one we won't tell him about beforehand. Grandparents may or may not, but as a way to coordinate what are gifts to our son, I have felt better and better about it over time. Knowing they wanted to give so much, another 529 wouldn't have been a good fit. Having the options will help him in many different scenarios, and we have been able to express ourselves in the way we are most comfortable with.
So, as I look to your situation, I can imagine in a mixed household with your stepmother, the idea of who gets what can be complicated. The need for their own account may be a compromise to keep the peace within the household. Of course, they should tell you that ideally, but I do see daylight in their insistence on keeping control. Or, they could be manipulative--can't lose the devil's advocate view on that.