User blog comment:B214/Volume 23 discussion blog/@comment-77.188.244.180-20170422191737/@comment-30924344-20170422192434

Ravel leaves her clan because she marries Issei who is already married to Rias becoming Issei Hyoudou-Gremory, so she has to take Issei's family name as Issei can't take hers for her to remain in the clan. With there being 4 Phenex children and her being the youngest, her parents can let one of their children marry a reincarnated devil although this is the result.