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:
In his response to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
Yes, a person may give own Zakah to a deserving adopted sibling (brother or sister) provided the latter is in the category of poor or needy even if the adopting person is her/himself not needy.
We must realize that Islamic adoption means taking care of a child, raising him/her within a family, educating, etc. but not legally making the child a son or daughter unless the adopting mother nurses the adopted baby from own breast for at least five full meals.
In other words, an adopted child remains stranger legally unless he/she becomes a nursing son/daughter.
Almighty Allah knows best.