How can I tell if my mother-in-law and sister-in-law deactivated their Facebook, or just blocked me?

[Average Person]

It's hard to tell deactivation and blocking and that's the point. Most people don't want to hurt other peoples feelings. If they blocked you and your brother also blocked you from seeing them via his FB settings it could still look like deactivation. It's honestly too hard to tell.

If you still see comments being made by the sister in law AFTER supposed deactivation, she more than likely blocked you. You can't post once it is done (unless you reactivate it).