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, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is absolutely not permissible for you to marry this girl until she becomes a Muslim and embraces the religion of Allaah and declares herself free from any religion other than the religion of Islam. The fact that she carries out some of the rites of Islam without declaring herself free from the false religions and disbelieving in them, then this does not benefit her with Allaah, and she does not become a Muslim just by this. The Prophet said: "Whoever says Laa Ilaaha Illa Allaah (None has the right to be worshipped but Allaah) and disbelieves in what is worshipped besides Allaah, then his money and blood become sacred and his reward is with Allaah.” [Muslim] Therefore, whoever does not disbelieve in what is worshipped besides Allaah, then he is not one of the Muslims.
What you mentioned about her fearing to disrespect or hurt her parents if she becomes a Muslim, is not a sound reason for her to stay on disbelief. If she really wants to be kind to her parents, then she should embrace Islam and call her parents (and family) to embrace it and abide by it, and by doing so, she will be happy in this worldly life and the Hereafter as well as them. Allaah guides whomever He wills to the straight path.
For more benefit, please refer to Fataawa 88531 and 84551.
Allaah Knows best.