What rights do siblings have for custody of a minor half sibling?

I am 20 years old, married and own my own home. I have 2 children ages 22 months and 4 months. I live in Iowa and I am a us citizen. I have a 2 year old sister I have never met. My father is in prison for ault and my stepmom is a drug addict though I cannot prove it. Before they had my little sister when I would visit the other children were always covered in bowel and urine and the house was disgusting. Both older children were mentally disabled ages 6 and 8 now and very scared all the time. I am trying to find my stepmom as it was a common law marriage and she no longer uses my fathers name. My father beat her children at one point and went to jail for 30 days. I do not know what her record looks like but I am guessing she has probably been caught at least once. I am worried my sister will grow up in that filth and become just like them and I know I could provide a great home. My husband makes a good income and has held the same job for 7 years and I start college soon