I'm sorry your mom no longer recognizes folks. But I am glad for your family that she is in such a great environment and has so much wonderful attention.
Remember, the idea of visiting your mom, for D, is at least partly for D's sake and not your mom's. D needs to see her, maybe for some personal closure. I have seen people visit folks who had dementia and the visit was as much if not more for the person doing the visiting as for the patient being visited. Also, remember that D may be pretty scared -- though she might never say so -- to see your mom; it will be sad and frightening for D, on some level, to see someone she has known her whole life who is essentially changed so profoundly.
You might gently tell D, "If you want to visit mom, that is wonderful of you. It means a lot to me that you want to see her. But if you decide not to see her again, I also really do understand and she would understand that too. Either choice is fine with me, and I want you to know you it's perfectly OK if you do want to come, or if you don't. This can be a tough time for her friends and family.
"I do want you to know, before you see her, that she very likely won't know who you are anymore. That's not the real her; that's the dementia and the medications, but it's how things are, and I am concerned that it will be painful for you to see her when she can't really interact with you like she used to, and may not even know you. But I understand that you might want to see her and just spend some time with her, just as she is now. If you want to visit and would like me to come along just for support I'd be very happy to do that. I can come into the room with you both, or just wait outside while you have your visit. I know that if she knew you were visiting she would be happy to see you."
I hope that helps some. I feel for her -- it's difficult for older folks to see relatives and friends growing weaker or developing dementia. I've seen it a lot among my late mom's friends and our relatives.