It is not unlawful to adopt a child. Upbringing and taking care of an orphan child carries great virtues. But merely by adopting a child the ruling of real child shall not be applicable on him rather it shall be necessary to observe Shariah purdah from him after he is mature. He shall get no share in the property. Yes, if a child is adopted while he is infant and he is breastfeed during the time of suckling then hurmat-e-raza’at (fostering relation) shall be established and he shall be a mahram for you and of your daughters and shall be allowable to live at home like a family member i.e. purdah shall not be necessary. However, he shall not be your heir in any case.
Allah knows Best!
Darul Uloom Deoband