Clara, have just re-read all your posts, as it seems most people are thinking you SHOULD tell your mother. I see that you have spoken about this suicide with your mother's brother...and he knows nothing about it...why don't you ask him whether it be appropriate to tell your mother?
I do agree, that if you are telling other members of the family, it may well backfire, and find a way back to your mother, leaving her to wonder why you didn't come to her about it!
My mother didn't know her grandfather had committed suicide, but her older sister did (that was how I found out). My mother was just 4 years old, when he died. My mother is 65, and had no fond memories, or any "fantasy" of her grand-father, in fact, her father had actually been disowned by the grandfather, so I actually did tell my mother, and she wasn't fazed.