I'm so sorry to hear about your step mom. Were you close to her? It's very difficult to lose someone suddenly. Heart attacks do that without anyone to blame. My husband died of a massive heart attack when out of town and I was on the phone with him fifteen minutes before he died and he was just fine. This, for your step mom, was probably stress related and also she could have had some heart disease that no one knew about. That's what happened to my husband.
I'm glad you can be there for your father. May prayers heading your way.
Sherrine