User blog comment:B214/Question and Answers/@comment-37.231.252.151-20130619053341/@comment-9339850-20130619073641

Well Ravel's mother can clearly see that her daughter is very much in love with Issei so im sure that is a reason why they let Ravel stay with Issei. Plus in the LNs Riser and Rias's fathers talked about how Issei beat Riser and felt it was good because it taught him a lesson, so from the beginning they seem to have had a pretty good impression about Issei, and im sure they also realize his power and potential and respect him for that as well, so in the end I don't think they are expecting anything substantial but are hoping for happiness in Ravel's life and are fine with her being with Issei as long as he cares for her as well.