Petitioning for half siblings in and outside of marriage

My wife (naturalized citizen) wants to petition for her half sibling, but unsure she is able because her half sibling was born to their mother outside of marriage after she fled from her husband due to domestic violence. My wife's father later shortly afterwards took the mother and daughter back and they lived in the same dwelling until his death. He also practiced polygamy and had children of which she would like to petition (I-130) for too. My wife currently has an approved I-130 petition for the mother. May my wife apply for her half sibling to her mother? May she also apply for siblings belonging to her father's second wife? There were no divorse decrees on either marriages. All half siblings are single and over age 21. Would a letter addressing the issue along birth certificates help?