It sounds like mom was very receptive to the sexual attention of your brother, and his intercourse with her brought them great pleasure. I can’t think of a better way for a mom to celebrate her son’s birthday than to offer him the opportunity to have intercourse with the same vagina that he came from. I’m wondering if your brother’s semen was discharged in mom’s vagina during their love session because that would make their intercourse an irreversible and fully consummated ince*st affair.