Citie doesn’t sound like any of the common nicknames used by grandchildren to call their grandmothers.

Here is a poll that says that among the 617 families who answered, 208 different ones were used. That’s quite a lot.

There is titi in the list. It is a common nickname in France, although not specifically for a grandmother. That or titie might be what your mother used anyway, especially if her grandmother’s first name contained t or ti.