All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, we ask Allaah to reward you for your good intention to become righteous, and we ask Him to enable you and make it easy for you to do so.
There is no doubt that marriage is the solution for you to become righteous; however, you should not hasten to marry that man. It might be that marrying him will be an obstacle for you in preserving your religion since he is not religious and he does not have a good moral conduct.
So we advise you to stay away from him since he is as you described in the question.
However, if he becomes righteous and you think that he will help you become righteous, and he shows signs which suggest that he is suitable for marriage, then you can inform your parents about your wish. But first, you have to perform the Istikhaara prayer and seek advice of those who know him better than you do. If your brothers are trustworthy, then you should not ignore their opinion as they might have known things about him that you are not aware of.
So if after all this, it becomes clear that it is suitable to marry him and your parents agree, all perfect praise be to Allaah. In this case you have to endeavour to please your brothers in order to maintain good ties with them.
However, if your parents object to you marrying him, then you have to obey them because you are obliged to obey them in what is permissible but you are not obliged to marry that man. Allaah may bless you with a husband better than him.
Allaah Says (what means): {But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allaah knows, while you know not.}[Quran 2:216]
In this case, you have to cut relations with him as it is forbidden for a woman to establish a relationship with a non-Mahram (i.e. a marriageable man).
Besides, if you had committed fornication with him, you are obliged to repent and it will not be permissible for you to marry him unless he repents, and provided you make sure that you are not pregnant from him.
For more benefit about the seriousness of adultery or fornication, please refer to Fatwa 85288.
Allaah Knows best.