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.
If this girl has these good qualities which you mentioned, then she is suitable to be a wife for you as there are not so many girls like her especially in that environment where you live. The fact that her family is bad is not a problem since she is herself righteous. The Prophet said: “Choose the religious one, you will prosper.” [Al-Bukhari and Muslim] The Prophet mentioned the religion of the woman and he did not mention the religion of anyone else. For more benefit, please refer to Fataawa 88338 and 2742.
Moreover, marrying her could be a means of saving her from that bad environment, and Allaah may make both of you a means to guide her family.
As regards your younger sister, you should try to protect her from those people and prevent her from socializing with them. You and your wife may cooperate in nurturing her and guiding her, especially that you mentioned that your wife is interested in children and teaching them in the mosque.
Finally, we advise you to perform Istikhaarah prayer in regard to this marriage; if there is any good for you in it, Allaah will make it easy for you, otherwise he will drive it away from you. For more benefit, please refer to Fatwa 81434.
Allaah Knows best.