Well, no answer IS an answer in a way, isn't it?
You know it got delivered, so if you are able to ascertain that she did read it, then I think she's made a clear choice and she no longer gets to tell you what she'd like to do about being a grandmother.
You have tried to put a bridge up, but those two are f*cked in the head if they think that they get to dictate the terms of any future relationship with your daughter. You abdicate all responsibility, you also abdicate all rights.
That isn't being vindictive, either. Showing unreliable behaviour to you now is a precursor of what you can expect when your daughter is older and can be badly hurt by adults who decide that their petty differences supercede the well-being of a vulnerable little girl. You have to protect her from that and you are.