This is a good example of the usage of the Preterite in Spanish, this verbal tense is used when you wanto to:
1. talk about past habitual actions, (adding used to in its different forms), to describe what used to happen or what was happening
2. describe what someone was doing when it was interrupted by someone (or something) else.
In this sentence:
The family used to visit the grandparents for their anniversary
We are talking about the first case, because this was an action that was taking place at that time in the past, and we can see the used to, as well.
Now, the preterite of the spanish verb visitar (to visit) with the 3rd person in plural ellos (they), which are The family, is visitaban. Therefore the complete sentence is:
