Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- As for adoption, in the sense of changing one’s identity and lineage, it is prohibited in Islam. However, it is allowed, even recommended, for Muslims to take care of orphans in the sense of providing both physical and spiritual care for them.
2- You are not allowed to live with your adopted brother as is not a mahram (unmarriageable) for you. So, you have to wear the hijab in front of him.
Responding to your question, Dr. `Abdul-Fattah Edris, Professor of Usul Al-Fiqh, at Al-Azhar University, stated,
It is settled that adoption in the sense of changing one’s name and lineage is prohibited in Islam. So, as long as you and this adopted brother are not birth siblings and you both haven’t shared the same mother’s milk, you are not allowed to live with him. He is not a mahram (unmarriageable) for you.
Given the above, you have to wear the hijab in front of him. Also, you can’t live with him as a brother.
Allah Almighty knows best.