She has role reversed so the sympathy is with her as the ill, struggling etc - when the reality is so different.
Not in RL, though. Only here in her MN persona, which is different from her RL experiences. She has created an online identity, with supporting characters, and she has used some of your real experiences and situations to define that.
I have learnt from some of the posts - about how people react to the situations presented and how I have been vilified.
But it's not you that has been vilified. You are identifying with a fictional character that DCs stepmum has created. It's not you.
Your exH may well be a waste of space, he may well be letting your DCs down, but that is unrelated to your DCs stepmums fantasy online life. Of course, his failings as a husband/partner may well be contributing to her need to create a "online life".
It's not unusual for regular posters on boards like MN to be posting from the perspective of a fictional character they have created, including charricatures of other people in their life. . What's more unusual is for one of those "charricatures" to discover it, recognise themselves and take the responses from other posters to heart.
This is not about you, whatever - it is a fantasy that meets the emotional needs of your DCs stepmum right now.