Has the husband been abusive to the grandmother, or
just to others? Will they be installed as legal
caregivers or are they simply moving in under the
implied condition that they help the grandmother?
Many states have requirements for legal caretakers,
some of which would be able to bar someone with a
history of abuse from assuming such a role. But, if
they are just moving in to help out, I'm not sure there
is any legal recourse that can be taken unless there
was a prior incident.
If there were, you could potentially file a restraining
order. I would speak with an attorney. Unfortunately, I
am not one.