How to desensitize yourself and stop having a PTSD reaction?

I have a history of PTSD from a sexual assault when I was 12. Although I’ve been to therapy and I’m not really triggered by that anymore (like I don’t have a PTSD reaction when I think about it anymore) I still get my same “PTSD” reaction to other lesser traumas.

Specifically, my MIL used to be sexually abusive to my husband as a child, and is highly verbally abusive to me and my husband now. My husband has cut her out of his life, however the rest of his family is always bringing her up and talking about how they want him to make amends with her, etc. (For the record, his family doesn’t know about the sexual abuse and he doesn’t feel comfortable telling them. They do know about the verbal abuse but they always condone it somehow...)

He has no intention of making up with her or even speaking to her, but whenever the topic comes up I instantly go into my PTSD stressor response—which is to have an awful stomach ache and painful bowel movement, sometimes diarrhea.

I used to get panic attacks pretty regularly (like shortness of breath and all that) but I’ve figured out how to control that. But this... this stomach ache stressor response, which is EXACTLY what used to happen to me when my PTSD was triggered... I have no idea how to control it. Every time her name comes up I have to run to the bathroom.

Please, any tips for how to stop being so literally traumatized by this woman who isn’t even in my life! Please DONT say things like “just don’t talk about her” or “cut out the rest of his family” because that’s not my decision to make. It’s my husband’s trauma, and it’s his family, and I want to respect his decision to keep ties to the rest of his family even if I agree they’re all horrible enablers.

I really need advice specifically geared towards handling the trauma-reaction, not handling the situation with his family.