Love and Correspondence Before Marriage
Question

I am a young man and fell in love with our neighbour’s daughter. This went on for two years without her knowing and without me telling her or anyone among my family, and without me meeting her. I was hoping that she would become my wife. Then I had a car accident that left me with extensive injuries and I became almost disabled. I was very worried because the dream that I had of marrying this girl seemed to be slipping away, then suddenly my sister asked me to tell her frankly what was worrying me, and I told her what was on my mind. My sister said to me: I am going to tell her of your love for her and after that she can choose. The surprise was that she also loved me, and we began to exchange messages of love via my sister (messages that were, by Allah, noble and chaste). After two years I proposed marriage to her and she agreed, praise be to Allah. After one year we got married and we lived happily, and during that period I only saw her from afar, because they are our neighbours and we would meet by accident. I never spoke to her except on the wedding night. I have heard that because of our love for one another we have committed a sin. Is this correct? What is the expiation for this sin?
Praise be to Allah.Firstly: 
We ask Allah to reward you for the calamity that befell you, and to increase the love between you and your wife.
Secondly: 
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. 
Thirdly: 
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.”
[al-Ahzab 33:32] 
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.