Well, not stop the sleepovers in their entirety but what age would you stop the mixed-gender sleepovers in the same room with cousins who are close and enjoy each others company?
How old is the eldest when you’d stop it, assuming they all still want to sleep in the same room?
How would you phrase it to the parents without causing tension?
FWIW - we have 6 cousins from two families who have a few sleepover a year.