What is the definition of a good stepmom? I have two stepchildren, ages 5 and 9.?

I have been in their lives for three years. Their mother was abusive, allowed only to have supervised visits with them for several months. The five year old calls me Mom. I am 50 this year and have a 15 year old daughter of my own. I am starting to resent having to spend so much time on my husband's children. He runs a small business and expects me to pick up all the loose ends plus more. I want to do the right thing and am really struggling with what that is and what is publicly expected of a stepmom in this situation.