I totally agree with you. If there is alcoholism, abuse, children being hurt, or mental illnesses that cannot be tolerated at a large family meal, then ok. I get it. I would probably have a conversation about them not being invited. Both my husband and I have had to deal with situations like this in the past, not inviting people, or having our parents not invite certain people, etc. So, I do understand that part.
I thought she was just referring to conversation topics, though and to me, that's not a reason to eliminate them from Thanksgiving when you have an otherwise decent (not perfect, but you're civil) relationship with them.