Estranged family member diagnosed with cancer a month before wedding

Title says it. My dad has a sister that has lied, manipulated and schemed all of her siblings nearly her entire life and at one point claimed she was nearly homeless and my parents had supported her financially only to find out she had taken all the money left from their mom’s will since she was the executor. My parents have forgiven her for their own wellbeing but keep her at arms length because of the cycles of abuse she puts them through.

To put it in short, she has attempted to guilt trip my parents since the beginning of the year for her to be invited to the wedding. I have no relationship with her and know she is an alcoholic (something she also told everyone she was sober of, but lied about for 20+ years and was even someone’s AA sponsor). Both of my parents are in full support of me not inviting her.

There have been multiple manipulative attempts from her to me directly on getting invited, nasty texts, and even going to the extent of saying “woah is me” and announced to everyone in the family group chat with my dad and her siblings that she was not invited to the wedding. My dad tried graciously explaining why and mending fences but she has now made the decision to ignore him the last 2+ months, to which, my dad is seemingly okay with. I can tell he is hurt by her decision, but he thinks her response is rather inappropriate. We all know she is doing this to punish him.

We are a month out and another aunt of mine texted me saying that she was disappointed in me that I didn’t invite my (abusive) aunt to the wedding. I sent a screenshot to my cousin (not related to the aunt that texted me the message or my abusive one) and asked what she would do. I found out that my abusive aunt has colon cancer that has metastasized to her lungs. She (abusive aunt) has told other family members just this last week and my cousin found out about it from her mom (the only aunt I have a healthy relationship with). The kicker is, my dad doesnt know about the cancer diagnosis. I would be lying if I said many of us are a bit skeptical because she has claimed similar things before and has lied to all of us for 20+ years, but this is something that could be very, very serious. My therapist (same as my mom’s therapist) truly believes she is a diagnosed sociopath because of her upbringing and behaviors from both adolescence and adulthood. We try to air away from using that word but it is something we make a mental note of because of how manipulative and emotionally difficult she can be.

I just dont know what to do. There are so many other little examples and details of this, but I cant sum it up in one post on reddit. I just feel horrible with all of this. I know she is hurt i didnt invite her, but I know she will continue to punish my dad if I don’t go through with inviting her. I dont want to see my dad’s relationship with her be decimated and there is an immense amount of regret if she does inevitably pass.

I just dont know how to make this better. We are still not inviting her but i dont know how to mend any fences. I feel partially responsible for the rift in their relationship.