&gt; But after her diagnosis, he ghosted her. She tried reaching out to him, but he just blocked her. That is when she realised my value in her life. She quit her previous job. I was totally unprepared for this kind of revelation. She swears that they never had any sexual intimacy. They wanted to end things with their spouses before committing. She also tells me that I'm the most important person in her life and she'd do anything to make it up to me. 

Let's unravel the bullshit she is spewing. 

She was planning on leaving you. He was planning on leaving his wife. Everything was going hunky dory until she got cancer. They were sexting or flirting or fucking. You won't know and it wont help the situation. The moment he caught wind that she had cancer, he noped the fuck out of that situation. 

You, on the other hand, were left holding the bag.

&gt; She tried reaching out to him, but he just blocked her. 

So she tried multiple times to make it work with him. She really wanted to be with him. He didn't (what a shock!).

&gt; That is when she realised my value in her life.
&gt; She quit her previous job.

My bro...you don't deserve this treatment. 
Stay with her if you must, but you will never be able to look at her the same way again. 
Ask yourself this, had she not gotten cancer, do you think the two of you would be cuddling together giving her the opportunity to tell you this? 
Chances are you would have been divorced by now. Maybe that's a hint.

Edit: Are you sure your children are yours?