Emc2, I gather that you don't feel D should be called "Grandma" the same as you because you put so much more effort into raising your sons and she was never much of a "mum" to them, etc. As a pp said, it must feel as if all your hard work is going unrecognized. I understand how that could hurt.
This isn't about what you did as a mum though or what she didn't do as a step-mum. This is about the gc. As others have said, she's their gm, too, because your ex is their gf. The kids will realize in time that she's not their "blood" gm, if that matters. As they hear stories of their dad's childhood, they will probably figure out, too, that you were more involved with him and his bros than your ex and D. But how they feel about anyone will depend, imo, on their own relationship with that person. So please just enjoy them when you're with them and don't worry about who else is there or what they call them.
In the future, however, I wouldn't invite ex and D to any events they wouldn't ordinarily expect to be at. Why cause yourself upset? It doesn't sound as if you've totally forgiven ex, so please don't try to force it. I can't imagine disinviting anyone, unless they had just done something horrid. But, from now on, please don't invite them to your personal celebrations such as your birthday.
It's not so much about "forgiving," anyhow, imo, as it is about letting go of the past. Kudos to you for having a good life of your own (job, relationship, etc.). Enjoy all that, and again, don't concern yourself so much with ex and D, one way or the other!