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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
First of all, you should know the following:
1) Your relationship with a marriageable woman, either a Muslim or a woman from the People of the Book or else, in the manner you mentioned, is not permissible. By establishing such a relationship, you are opening a door of temptation to yourself and a cause of committing a sin. Therefore, you should cut the relationship with her and you should not talk to her except according to the necessity while lowering your gaze, avoiding being in seclusion with her, touching her and the like.
2) After you discovered that your wife has a deviant tendency to her female friend, you are obliged to advise her and frighten her with the punishment of Allaah if she pursues and follows her deviant desire. In order to prevent the sin, you should prevent your wife from staying alone with her. It is forbidden for her to sit with her or look at her if she fears to be tempted by her.
3) In principle, it is permissible for you to marry this woman of the People of the Book, but since it is feared that your wife and her will be committing a sin if you marry her, as they would be meeting and staying together longer, then, in our view, this makes marrying her forbidden. As regards calling her to Islam, this could be done without you marrying her, and there are many safe ways of calling her to Islam.
Allaah Knows best.