I am a carer also and I agree with that point completely. I also have seen what happens when family members are looking after those that they dont like, have no feeling for or have some grievances regarding historical relationship dynamics.
Everyone has this rosy ideal picture of children supporting their parents (or in this case step mother), and wonderful if it happens like that. But in the real world things dont always happen like that and not all families are the same.
I gave up work to move my Mum in with my family into a purpose built annexe in our garden when her dementia became too advanced for her to live at home with carers. I had the support of my WfH DH, my 2 school age DC, a local village befriending group and daily carers. And it was despite the help, on going relentless 24/7 pressure. But my situation was unusual and I was lucky to be able to do it and I wanted to do it.
But how many people on this post would be in the situation at home, emotionally or financially to do that? Just because the OP & her full siblings have already helped out with her dad (cant nb re her mum), she & her family may not be in a position to do so currently with her SM. We dont know circumstances and life moves on, things change.
Also sometimes things happen in families which mean that that relationship has been killed off or damaged beyond repair. What goes around, comes around. And in those cases its much better to have carers in to help out. The carers arent weighed down by historical issues and develop a very different type of relationship. Most of my patients will talk to me about things they would never dream of mentioning to family or even close friends.
I have a relatively good relationship with one of my half brothers from my dad's second marriage, but I never expected him to help with my Mum. She was my responsibility.
The OP's half sister is the one responsible for her own mother. She is probably panicking abit because shes just realising its on her shoulders & it is a heavy weight to bear. But she definitely shouldnt be going round the family complaining about the OP to guilt her. If she & her mother were never close with the OP why does she expect OP to help look after her mother?
Yes the cheating and divorce and the step family happened a long time ago, but emotional damage from a divorce doesnt just disappear. It stays with the children into adulthood. I think its unfair for people to assume the OP should help her SM if she doesnt want to.