My brother has 4 children. His elder son is about 12-years-old. He is having 3 boys and a girl. My brother engaged in Thableegh activities. But he now took a matured girl around 12-14 years of age for adoption. It is permissible see that for me or my brother at least even for his son. Without suckling how to become Mahram.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

This girl is a marriageable girl for all of you, so it is not permissible for any of you to look at her, or to be alone with her, if she is not a Mahram for you by suckling, family lineage or related by marriage. It should be noted that if what is meant is that your brother has adopted her (claimed her as his natural born child), then such practice is forbidden in Islam. Indeed Islam came to forbid adoption, Allaah says (interpretation of meaning): {Call them (adopted sons) by (the names of) their fathers, that is more just with Allaah.}[33:5]. But if what is meant by adopting her, is taking care of her, looking after her affairs, raising her, bearing her expenditures (while tracing her back to her original father), then this is a very good act, and an act by which one gets nearer to Allaah. But it is obligatory to be cautious since she is a marriageable girl to you.

Allaah knows best.