So sorry this happened to you, Poly! What a shock it must have been after having such a loving relationship with dd! My heart goes out to you.
I suppose it was a shock to her and sil, too, when you "shut the bank of mum and dad." But, imo, they should have realized that you couldn't finance their lives forever and that it was time to stand on their own 2 feet, period.
It sounds as if it was mostly sil who was upset. So dd may simply have decided to stand by him, as spouses often do. I'm sorry this has led to your being estranged for so long from her and your gs. Frankly, I don't understand why she couldn't have continued to defend him without cutting you and dh out so completely.
Perhaps there were some harsh words exchanged when all this first happened? Some angry remarks and insults that are hard to take back? Perhaps that's why she's taking sil's side in such an extreme way?
Anyway, I don't think you should continue to send cards. They probably don't get to gs and may even be tossed, sad to say. Also, since dd and sil have chosen to go nc (no contact), the cards may just antagonize them further, which could prolong the estrangement. I much prefer ninathenana's idea.
Same with the texts. Unless she replies (it sounds like she doesn't), then, imo, they need to stop. Though they are loving and caring, they may actually be making things worse.
And I wouldn't worry about letting dd know the door is always open. You have already told her that. If you feel more comfortable, maybe send her one more text saying that and adding that you won't be contacting her anymore since she doesn't seem to want it. She may not even read it, but it might make you feel better about ending all contact for now, until she's ready to walk through that open door.
(((Hugs)))