Answer:
visitamos
Step-by-step explanation:
In order to change this from "my brother and I to visit our grandparents every Sunday" into "my brother and I visit our grandparents every Sunday" we would need to change (conjugate) our verb, visitar (to visit) into visitamos (we visit). My brother and I could be rewritten as we so we would conjugate it as if we were saying we/nosotros.
Another option (one I don't think your teacher was looking for) is to reword the sentence into: "my brother and I go to visit our grandparents every Sunday" which would translate into "mi hermano y yo vamos a visitar a nuestros abuelos todos los domingos."
Hope this helps! :D