It also could come down to exactly how old you want the grandmother to be. If you're not gonna have her be an elemental for any specific reason, you probably don't need it. Overall wizards love pretty long. Mcgonagall was like 90 and she looked like she was middle-aged still. I have written some family members who were around 150 when they died.
You could also have her take some sort of special potions regimen or just have her be really healthy?
As far as the Parseltongue app, there are some guidelines with that, as far as personality and blood status go, in that case, if you wanted a Parseltongue relative, they should be there for a reason, I think. If you have a good one, perfect! But overall, and an admin can correct me if I'm wrong, we try to keep uncommon occurance (elementals, twins, orphans, etc.) lower in numbers if possible to follow reality.
But if you were thinking of an sf for a character, definitely ask around and you can get a lot of advice.