I have a lot of moles and got sunburned a LOT as a kid and even as a teen. I don't get them much now because I don't go outside that often (lol). If you have fair skin, it's a thing that happens very commonly. I've even gotten sun poisoning from a sunburn before. There is not a "normal" amount of sunburn to get.
Your grandmother getting melanoma does not mean that you will. You also remember that she has 86 YEARS of skin damage under her belt. In addition to that, it doesn't have to happen because of sun damage. Someone with no sunburns in their life could get it.
An increased risk isn't a guarantee that you'll get it.