User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-30924344-20170826132209/@comment-10330822-20170827212545

No, only Ravel fell in love with Issei at that time. Rias was in love with him before hand, hence why she wanted to give her virginity to Issei before she was forced to marry Riser. Issei's rescue of her just cemented her feelings. Rias probably fell in love with Issei after seeing how kindly he treated Asia and her. Rias had always been pursued by others because of her beauty and position, but Issei seemed to genuinely care for who she was.

Ravel on the other-hand fell in love with Issei because he reminded her of those heroes from stories that she was read as a child that came in to protect and save the princess, aka a Shining White Knight or Prince Charming.