silentbuddha, in reading what your mom wrote to your daughter, I don't think she was trying to preach to her. I think your mother believed that your daughter believes in Jehovah (and maybe even believed that you believe in Jehovah, even if you have told her multiple times that you no longer believe in Jehovah), and that part of the reason why she believes such is due to her hoping that you and your daughter will become saved and enter the paradise New Order.
In my own situation it was (and still is) difficult for my mom to accept that I am now an atheist. For years there were times when she taught I must still have some belief in Jehovah or that seeing certain things happening in the world must have caused me to believe in Jehovah again. She tells me she still hopes I will change my thinking and return to the organization and to worship of Jehovah, before it is too late (from her perspective). She really, really believes the great tribulation is extremely near. Most of the times when I tell her "I am an atheist" (in order to make a point with her that I will never become a believing JW again nor attend the Kingdom Hall again) I see great emotional pain in her face and I hear it in her voice. She doesn't want to be confronted with the idea that I am a convinced atheist - even though it was 10 years ago (and thus long ago) when I became an outright atheist. She loves her son (me) and remembers when I was a ministerial servant for years and that I even gave some Sunday sermon public talks, and was a regular pioneer. She also tells me I am a good son. My mother remembers those things about my past like your mother remembers your daughter's past love for Jehovah and your daughter's former worship of him.
Your mother wants your father to become healthy and she believes in the power of prayer to Jehovah God. She also apparently believed that if your daughter prayed to Jehovah for grandpa (your father) that it might help grandpa. As a result, she wrote the letter to you daughter - mostly for the benefit of your father (not to preach to your daughter and not to lure your daughter to becoming a JW). That is what I think she was thinking, but I might be wrong. Notice in her reply to you she says she thought your daughter still believed in Jehovah and possibly prayed to him. At the time your mother also said she was still hoping you would return to Jehovah. I think she meant all of that.