If you are already at the school with younger child, you should engage with the admissions office directly, they will likely tell you whether there is a chance to get in 6th grade or if waiting for 9th is better. The single sex schools all have more spots in 9th grade as they lose a bunch of kids to co-ed schools (and some boarding as well). 6th grade entry is more attrition based but usually still at least 4-6 spots. And siblings typically do get special consideration though of course have to be qualified.