Yep. If the grandparents want access to their grandchildren, they must be nice to the parents.
Absolutely. No child was ever permanently damaged simply because their grandparents lost access. Personally, even though these people are not toxic, I'd be limiting my child's time around them. I need my kids to have good role models.
Maybe not, but they may have missed out on having another person in their life to love and support them.
Obviously if someone is toxic, they don't deserve to be around the grandchildren. But some people use kids as leverage to get what they want. My brother & SIL's definition of "be nice to the parents" is "do favors for us whenever we ask for them, and give us money when we ask for it." If you don't do these things, you're cut off from contact with the kids. It's the reason my mother hasn't been allowed to see her grandchildren in over two years. Children are not a "trump card."
Maybe the words "trump card" are a sticking point. It rather implies that games are being played.
It is unfortunate that your sibling and his wife are selfish parents, but the principle stands - the only way to have a relationship
with a child is to maintain a relationship
with that child's parents. If the kids in this case are hurt, it's not because of the lack of grandparents in their lives, it's because their parents are less than ideal.
In other cases, it is best for children to have a home and parents that are, for the most part, free of stress and anxiety. If this means limiting and controlling contact with stress inducing people, grandparents or not, then so be it. If a stress-producing person can be encouraged to behave better in hopes of spending more time with their grandchildren, well that's a good outcome too.