I think you already know the answer to your question, so let me help you with the part that’s getting in the way of your acting on it. You sound clear about wanting to cultivate relationships with your extended family, but you’re struggling to embrace the fact that, as an adult, you’re free to choose how to live your life.
Children don’t have this freedom, so if a parent chooses to alienate a child from a grandparent, the child misses the opportunity to form what might be a meaningful relationship. Studies show that when kids are close with their grandparents, they’re less likely to become depressed as adults. Grandparent-grandkid relationships can be a source of comfort, learning, and fun, even with those grandparents who had a challenging relationship with their own child many years ago. Many grandparents will relate to their grandchild in a different way than they related to their own child.
Of course, parents understandably limit or block access to a grandparent under certain circumstances. Maybe the grandparent is an addict, a sex offender, or otherwise creates an environment that is physically or emotionally unsafe for the grandchild. But sometimes, in an attempt to work through their unresolved childhood wounds, parents use their own children as pawns in a battle that has nothing to do with that child—and while the war is waged against the grandparent, the child becomes a casualty.
What’s happened here is that a conflict between two adults (your father and his mother) has been superimposed on a child (you). This sometimes happens between divorced parents, too, when a child is made to feel that having positive feelings toward the other parent is not okay, because of a battle going on between the adults: Can I hug Dad warmly in front of Mom? Do I have to edit out all references to Mom in front of Dad because he feels betrayed by her? You seem to be in a similar predicament now: Can I tell my dad that I want to spend time with the woman who hurt him but who has done nothing to me?
Your father may have very good reasons for choosing not to be in contact with his mother, but those reasons aren’t yours and never were. He doesn’t realize that instead of keeping you safe by separating you from your grandmother, he’s left you feeling unsafe because there’s no room for you to have a self that’s separate from his.
Now is the time for you to take ownership of yourself as a fully formed adult, and the first step is to recognize that, without intending to, your father has been asking his children to ease his childhood pain. I don’t know the nature of his conflict with his mother, but it sounds as if, at the very least, he felt she couldn’t acknowledge—or help soothe—that pain. By forbidding you from meeting her, he’s asking you to see what she couldn’t, which is how much pain she caused him—and in this way, he finally gets the validation he has needed. But nobody can heal this for him but himself. And without even realizing it, he is hurting his own child, unable to see her pain.