[zzhk]

She is just trying to tell Rias that because it is Ise's wish to have a harem, it is Rias' responsibility, but without explicitly naming Ise.

"Your father is also under my management. Strong attractive men getting stolen by other women happens all the time."

Of course, the ideal situation is to have the husband under control with no other wives. I'm sure Venerana Baal snapped up old Gremory as quickly as she could. But Rias is too late for that: Ise has already expressed his wish for a harem. Asia and Akeno are unshakable at this point. Hence, Venerana is telling Rias to go for the next best thing: harem manager. Stand above all the other girls and show them who is boss.