Its ok to marry a 10 year old girl, but its wrong to have sex with her before she is 20. Its ok to marry a step sibling. Its not ok to marry a step daughter. Its ok to marry an adopted daughter but only have sex when shes 20 or older. The issue with a step daughter is its wrong to have sex with someone and their parent or someone. Like its wrong to marry a woman and have sex with her and then after she dies to marry your step daughter from the wife that died, because the step daughter is closely related by blood to someone you had sex with. Likewise its wrong to have sex with someone and to sometime after have sex with their sibling, aunt/uncle, or niece/nephew. But in the case of someone adopted, with no close blood connection to you or any of your partners, i see nothing wrong with adopting/buying a nonrelated daughter and taking her for yourself as a wife if she consents.
Exodus 21 "And if a man sell his daughter to be a maidservant, she shall not go out as the menservants do.8 If she please not her master, who hath betrothed her to himself, then shall he let her be redeemed: to sell her unto a strange nation he shall have no power, seeing he hath dealt deceitfully with her.9 And if he have betrothed her unto his son, he shall deal with her after the manner of daughters."
I interpret this as endorsing adopting someone's daughter and marrying her. But even if not, i still think its ok to marry an adopted daughter thats nonrelated. Because in my view, blood parents will always be the true parents. I do not recognize the validity of adopting children as any true change of parenthood. The true parents will always be the biological, and nothing can change that. So if you adopt someone as your child you may convince yourself they are your child but they are not in any way shape or form. They are merely bought by you. They were sold by the parents or the state to you. But they are not bound to you as the true parents they will always be bound to first and foremost. Thus marrying adopting kids and having dex when they are adults is fine to me since i do acknowledge the legitimacy of adopted children.