I used to have panic attacks at the idea of my NM coming to my state, at the idea of going to hers, etc.
A good therapist specializing helping adult children of alcoholics treated me for PTSD.
worth 10x what I paid her.

I also suggest the books "Healing the Child Within" and "Co-Dependent No More".
Both were very helpful for me. The first describes a lot of family dynamics... which might give you more insight into your DH's relationship (or lack of) with his sister.