Some families prepare their ancestors’ ceremonial food. what is the indirect object?

There is no indirect object.

Indirect object would be for example here:

Some families prepare their ancestors’ ceremonial food for their children.

 
their ancestors’ ceremonial food is a direct object and there is indirect object.

Alternatively - it would be "their ancestors" if there was no apostrophe (but the style would still be ugly)

In june, we will have been married for eighteen months. **