Marylane please be careful. You do not say whether your DD has husband/partner support, or how he feels about you. If she is alone and on medication, with the problems you have described, she is not going to be considerate, reasonable or rational. I understand how you have tried to compensate for the loss of her father; I did the same with my DD over her turbulent childhood and my eventual divorce from my husband. The result is that I am constantly hurt by expectations that are not met because I have created a situation where my needs are unrealistic! It is hard to step back or to accept that we can't have what is right. It is hard to cope with inconsiderate and unreasonable behaviour. But you only have to look on this website to realise that you are most certainly not alone!
Sadly, we are usually responsible for how our children treat us and facing that isn't easy. If you've let DD walk all over you for years, as you say, then she's not going to change now! Is she having counselling? Are you able to just sit with her and talk to her and ask how you can help her? It may be absolutely the last thing you want to do, but if it helps improve the situation, it may be worth it.
Just do a normal size card with lots of love and wishing him better soon in it! And maybe an appropriate small present from Grandma. DGS will know you love him. As everyone has said, he's growing up and this situation will sort itself out in a few years. It's hard, but be patient. And be good to yourself too - you've done nothing wrong other than love them!