Welcome to the Dementia Support Forum
@ASKW. I am sorry to read about the issues that you are having with your FIL which must be so concerning for you all. It would help if you would give some more information about what form this aggression takes. For example, you have not said if the aggression is physical or verbal, or if there is anything in particular that causes the aggression.
if it is physical aggression you would need to ensure that your MIL is safe from any attacks as she needs to be protected. Verbal aggression can also be upsetting and cause mental distress to the person on the receiving end.
It might be useful to contact your FIL's GP to explain the issue as there are medications that can reduce aggressive behaviour in people with dementia.
