Praise be to Allah.
We ask Allah to reward you for the calamity that befell you, and to increase the love between you and your wife.
A person cannot be blamed for love that he does not cause, such as if he sees a girl by accident and his heart is filled with love for her, but he does not do anything haram (impermissible) such as looking repeatedly or shaking hands or being alone with her, or exchanging emotional words with her. As for the love that stems from repeated looking, haram mixing or correspondence, the one who does that is sinning to the extent that he does haram things in his relationship and love.
Correspondence between the sexes is not permissible, because it provokes temptation and usually results in evil. If a man corresponds with a non-mahram woman (a woman whom one is permitted to marry according to Islamic law) in letters that are not seen by anyone else, that leads to many evils. Islam forbids a woman to be alone with a man who is not her mahram because of the fitnah (temptation) and bad things that result from that, such as attachment and the desire to look and touch, etc. All of this results from the man talking to the woman in these private letters or conversations, especially if they are young and at an age when desire is strong.
Shaykh Ibn Jibreen (may Allah preserve him) was asked: What is the ruling on correspondence between young men and young women, if this correspondence is free from immorality, love and desire?
He replied: It is not permissible for any person to correspond with a woman who is not his mahram, because of the temptation involved in that. The person may think that there is no temptation, but the shaytan (devil) will keep trying until he tempts him through her, and tempts her through him. The Prophet (peace and blessings of Allah be upon him) told the one who hears of the Dajjal (known to non-Muslims as the anti-Christ) to keep away from him, and he said that a man may come to him as a believer but the Dajjal will keep trying until he tempts him and confuses him.
There is a great deal of temptation and danger in correspondence between young men and women which means that it has to be avoided, even though the questioner says that there was no love or desire involved. End quote from Fatawa al-Mar’ah, compiled by Muhammad al-Musnid, p. 96.
The woman is forbidden to speak softly to one who is not permissible for her, as Allah says (interpretation of the meaning):
“... be not soft in speech, lest he in whose heart is a disease (of hypocrisy, or evil desire for adultery) should be moved with desire, but speak in an honourable manner.”
And it is haram for a man to enjoy listening to the speech or voice of a non-mahram woman.
Making your sister the go-between in passing on the letters is another evil, because that is encouraging her to engage in this kind of activity.
Based on this, what you have to do is repent to Allah, and do a lot of righteous deeds, but you do not have to do anything else.
And Allah knows best.