If this question is asking what is that largest Marine Corps base "area wise" it is the Marine Corps Air Ground Combat Center Twentynine Palms located north of Twentynine Palms and Palm Springs,...
Camp Pendleton and Camp Lejuene are going to be the two largest. Parris Island and MCRD San Diego are going to be two more. At that point it becomes smaller places nearby, including various Marine...
All Marines are brothers/sisters so over 200,000 siblings haha. but i know of twins that both served together and in Saving Private Ryan i think it was 4 brothers so i would guess up to 5 siblings...