sisterhood in Islam is something very special that a sister who has come from a life as a non-Muslim and embraced this beautiful religion of Islam notices immediately. Her sisters in Islam treat her in a manner that no one has ever done before, not even her family.
So what makes this relationship so special?

She Loves Them as Sisters For the Sake of Allah
It doesn’t matter what your skin color is, the language you speak, or the status of your family, or whether you are rich or poor, in Islam we are all united like a single family. Our belief in Allah united us: “The Believers are but a single brotherhood…” [Noble Quran 49:10]

Prophet Muhammad (peace be upon him) said: “There are three things that whoever attains them will find the sweetness of faith: if Allah and His Messenger are dearer to him than anyone or anything else; if he loves a person solely for the sake of Allah; and if he would hate to return to Kufr after Allah has rescued him from it, as much as he would hate to be thrown into the Fire.” [Bukhari and Muslim]

The Status of Two Who Love One Another For the Sake of Allah
There is great reward in loving one another for the sake of Allah: “Where are those who loved one another for My glory? Today I will shade them in My shade on the Day when there is no shade but Mine.” [Muslim]

Prophet (peace be upon him) said: “A man went to visit a brother of his in another village. Allah sent an angel to wait for him on the road. When the man came along, the angel asked him, ‘Where are you headed?’ He said, ‘I am going to visit a brother of mine who lives in this village.’ The angel asked, ‘Have you done him any favor (for which you are now seeking repayment)?’ He said, ‘No, I just love him for the sake of Allah.’ The angel told him, ‘I am a messenger to you from Allah, sent to tell you that He loves you as you love your brother for His sake.'” [Muslim]

And we are encouraged to tell one another, to promote a loving bond between each other: Anas said that a man was with the Prophet (peace be upon him) when another man passed by. The first man said, “O Messenger of Allah, indeed I truly love this man.” The Prophet (peace be upon him) asked him, “Have you let him know that?” He said, “No.” The Prophet (peace be upon him) said, “Tell him.” He caught up with him and told him, “Truly I love you for the sake of Allah,” and the man said, “May Allah love you who loves me for His sake.” [Abu Dawud]

Effect of Love for the Sake of Allah on the Life of a Muslim Woman
It is important that we greet one another in the proper Islamic manner:
The Prophet (peace be upon him) said: “By the One in Whose hand in my soul, you will not enter Paradise until you believe, and you will not believe until you love one another. Shall I not tell you of something that if you do it, you will love one another? Spread Salam amongst yourselves.” [Muslim]

And when one of us is hurting, oppressed or suffering in some way, we all feel it:
“The believers, in their mutual friendship, mercy, and affection, are like one body: if any part of it complains, the rest of the body will also stay awake in pain.”