My granddaughter has a boyfriend and her mother (my daughter) is dating the boyfriend’s father. Is this considered incest? Or is it incest only if they are related by blood?
Leviticus 18:1-20 contains a long list of forbidden relationships, but the ones you describe are not included there. Therefore, the relationships are not incestuous. Even if your daughter married the man she’s dating there would be no blood relationship between her daughter and her husband’s son, since neither one shares a biological parent with the other. Since they could be considered step-siblings they would have an interesting story to tell in explaining their relationship, but that’s all.