sometimes, for cradle Catholics (or any other religion), it is hard to understand all of the ramifications that come with converting. To many family members, it can be seen as a betrayal of the family, and it’s heritage.
I was in a similar situation as the OP.
My grandfather was a Presbyterian minister for many many years, and at the time of my conversion, he was in his mid 80’s.
My joining the Catholic Church was a big deal, especially since I used to play the organ for his church, and take him to visit his parishoners. (he was blind)
In my situation, I felt it was necessary to be 100% honest with God, my family, and myself.
I did tell my grandfather, but I did it delicately, and over time. It started off with simple conversations, and ended up getting deeper and deepr until I told him what I was doing.
I can’t say he was jumping for joy, but he understood. He could also tell that it was something that I wasn’t taking lightly, and had done some real soul searching, and he commended me for that. After he accepted it, it became much easier to deal with other family members.
Unfortunately, he passed away before I actually joined the Church, (I don’t think I killed him…

), but he was OK with it, and that meant alot.
I knew he was watching me and smiling at the Easter Vigil when I made my Profession of Faith.
I guess what I am trying to say is, don’t sell your grandmother short. She may be more understanding that some family is willing to acknowledge. Just be tactful when you do it.