If you went with the Joint Tenants option, It'd get messy if your mother died before your grandmother.
You'd be back at square one. The grandmother would re-acquire the whole of the property.. so when she eventually dies, (assuming there is no change to her will) this would be split between her two daughters.. your aunt would get one half and your mother's half would be split between yourself and your siblings (if any) your mother's partner, (if any)... etc.
Hope everything turns out ok. Family squabbles are annoying.
Or more likely, if your mum died before the grandmother the aunt would get everything.