Edit - Removed post because there were too many specifics.
Long story short, my narcissistic, toxic father in law has cancer. He treats everyone cruelly. My husband left home for 3 years (before meeting me), because of his treatment. He is just a horrible horrible man, and my husband knows it. But now because he has cancer, he is putting his dad on a pedestal.
I only want to support my husband, but I cannot pretend that I care for this man, who has caused me to go into depression.
That sounds like quite the precarious situation especially if the gentleman involved can be defined as a “Thorny rose”. In this particular case due to the troublesome nature of the relationship, I suggest that your level of involvement should stay relevantly the same: Be emotional supportive and available for your husband. E.g. Listen to his worries, let him know your there whenever he is ready to speak, provide physical comfortable – hugging, handholding – simple familiar welcoming touch goes a long way. Or provide the occasional distraction if needed – Go walking together, do hobbies, jokes and talk about anything in general really. It’s the simple things that assist and matter.
But no matter the decision, he decides to select, such as:
Direct involvement: Having some form of deeper involvement e.g. Attending treatments, appointments and etc.
Barrier involvement: Only wanting to receive the news instead and provide limited emotional support to the father but greater support for other family members instead.
Either choice whether limited or direct involvement is selected, he will receive some form of necessary closure from this.
Be directly involved with your husband but keep that necessary barrier between you and the father-in-law. Limit or avoid all direct contact with the individual as much as necessary. You can still be there for your husband but you need to protect yourself as well.
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