14.- My mother and I __________ (not visit) our grandmother today.

The word 'today' indicates a current action that may not be happening or may be planned. If it's a statement about the present, present continuous is appropriate. If it's a statement about a missed opportunity or a future plan that is not happening, simple present or present continuous can be used. Given the context, the present continuous is most likely intended to express an action that is not happening now.

14. My mother and I are not visiting our grandmother today.