All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
If that woman was unjust to you and you spoke to her cousins about it with the intention of getting advice and consultation or to seek help to change this evil, according to the need without any transgression, then this is not forbidden backbiting. Please refer to fatwa 81748.
This is also true if you had spoken to them with the intention of relieving your stress because of the harm and psychological pressure that you suffered because of her injustice to you, provided that you said the truth and related what happened without lying.
Shaykh Muhammad ibn Saalih Al-‘Uthaymeen said about the interpretation of the verse: (what means) {And do not spy or backbite each other...} [Quran 49:12]:
“But is it permissible or not to do this if the person's intention is to ease the burden of grief and pain in his heart, such that he relates what happened to him to a friend who cannot remove this injustice from him but he consoles him?
It seems that it is permissible, according to the general meaning of the verse: {Allah does not like the public mention of evil except by one who has been wronged.} [Quran 4:148] And this happens very often; a person is often harmed by his money being taken unjustly and the like, so he goes to his friend and says: so-and-so did this and that, intending to ease the pain and grief that is in his heart, or he speaks about this with his children, or with his family, or with his wife, and the like; there is nothing wrong with this, because the oppressor has no sanctity for the oppressed.”
However, if you talked to her cousins for no benefit, or if you exceeded the need, then this is backbiting and you have to repent from it by not doing this again, regretting having done it, and being determined not to do that again in the future, but you are not obliged to tell her that you had backbitten her if you fear any harm or if this would again lead to dissention between you and her. It is enough that you repent to Allah. For more benefit, please refer to fatwas 88893 and 251593.
As for the woman who informed her of what you said, then this is Nameemah (tale-bearing). There is a severe threat for the one who commits it, and she must repent to Allah. Huthayfah, may Allah be pleased with him, narrated that the Prophet said, “No tale bearer will enter Paradise.” [Al-Bukhaari and Muslim]
You have done well by inviting that woman to start a new page with you. We advise you to be kind to her, because doing good to the person who wrongs you is one of the most important causes for changing enmity between you into affection, as Allah says (what means): {And not equal are the good deed and the bad. Repel [evil] by that [deed] which is better; and thereupon the one whom between you and him is enmity [will become] as though he was a devoted friend.} [Quran 41:34]
For more benefit, please refer to fatwa 84351.
Allah knows best.