What is narrated about the virtue of sponsoring orphans, and the relationship of the orphan girl to the one who adopted her after she reaches puberty
Question

If a man adopts an orphan girl, and she has no ties of kinship with him, is this girl obliged when she grows up to observe hijab in front of this man? What are the matters that he must bear in mind with regard to the issue of adoption, taking into consideration the fact that he did not give her his name? All the man wants is to attain the reward mentioned in the hadeeth of the Prophet (blessings and peace of Allah be upon him): “Whoever raise two girls until they reach puberty and get married, he and I will be like these two on the Day of Resurrection” and he held together two of his fingers. Narrated by Muslim and at-Tirmidhi.
Praise be to Allah.Firstly:
The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever takes care of two girls until they reach puberty, he and I will come like this on the Day of Resurrection” – and he held his two fingers together.
Narrated by Muslim (2631). According to a report narrated by at-Tirmidhi (1914): “Whoever takes care of two girls, he and I will enter Paradise like these two” and he gestured with his two fingers.
The commentators interpreted the virtue mentioned in the two hadeeths as referring to daughters and sisters. Imam Muslim narrated in the previous hadeeth, “Whoever takes care of two girls…” in the chapter on “The virtue of treating daughters with kindness.” And Imam at-Tirmidhi narrated it in the chapter on “What was narrated about spending on daughters and sisters.”
The one who takes care of his daughters and sisters properly will attain the virtue mentioned in these hadeeths, which is being with the Prophet (blessings and peace of Allah be upon him) in Paradise.
But with regard to the virtue of sponsoring orphans and the great reward for that, Muslim (2983) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The one who sponsors an orphan, whether it is a relative of his or not, he and I will be like these two in Paradise,” and Maalik pointed with his forefinger and middle finger.
An-Nawawi (may Allah have mercy on him) said: With regard to the words “whether it is a relative of his or not”, the one who is a relative of his is one who is related to him such as his grandfather, mother, grandmother, brother, sister, paternal uncle, maternal uncle, maternal aunt, paternal aunt, and other relatives. And the one who is not a relative is one with whom he has no blood ties. End quote from Sharh Muslim by an-Nawawi.
It is proven in at-Tirmidhi (1918) that Sahl ibn Sa‘d (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “I and the one who sponsors an orphan will be in Paradise like these two,” and he gestured with his two fingers, meaning the forefinger and the middle finger. Classed as saheeh by Shaykh al-Albaani in Saheeh Sunan at-Tirmidhi.
Secondly:
In the answer to question no. 126003 we discussed the issue of adoption and said that it is of two types, permissible and forbidden.
With regard to the permissible type, it means looking after the orphan and taking care of his affairs, without attributing the orphan to the one who adopts.
In that answer, we also stated that when the girl reaches puberty, her sponsor is regarded as a non-mahram to her, so it is not permissible for her to uncover her face in front of him or to be alone with him, but she may uphold ties with him by greeting him and asking him how he is, in return for his kindness and generosity towards her when she was small.
It says in Fatawa al-Lajnah ad-Daa’imah: It is not permissible for you to stay with this man who adopted you, and it is not permissible for you to uncover your face in front of him, because he is a non-mahram to you and adoption does not mean that you are related. … But there is nothing wrong with greeting him verbally without shaking hands, praying for him and thanking him for his kindness.
End quote from Fatawa al-Lajnah ad-Daa’imah, 20/363-364
And Allah knows best.