10:30 pm to late

My husband thinks 10:30 is a good bed time for a 5 month old because our 10 year old goes to bed at 10:00 pm. But our 5 month old is up ever day at 7 or 8 am which means she isn’t getting enough sleep at night she only sleeps 3 hours straight and once ever blue moon 5 hours straight so that means she is getting roughly 8 hours of sleep in total not enough for a baby. I’m trying convince him she needs an earlier bed time and not the same one as her big brother. Does anyone else think 10:30 is to late? Or that 8 hours total sleep for a 5 month is to little? He thinks it’s not a Problem because she is hitting all milestones and is even sitting without assistance idk ladies maybe I’m just over thinking it.