Thing is, it's not just siblings that affect things. It's also down to all of the other admissions criteria - usually "looked after" children/special needs naming that school, then sibs, then distance. Even if you know the number of siblings you don't know whether three of those families may move away from the area, or put their kids in prep schools, for example. You don't know how many adopted kids or children in care, or with specific additional needs there are and, crucially, you still don't know how many children live just a fraction closer to the school than you do.
Sibling numbers do have a big impact (assuming sibling priority is part of the admissions criteria) but it's not the only factor and the only way you're going to know whether you will get a place is to wait until the day they release admissions results. That really is all there is to it.
By all means you can try asking the school - some will tell you what they know, others won't - but it won't really make a difference. If it is the school that you really want your child to go to, then put it at number one on the application form. Whichever school you apply for, I'd avoid giving your child an impression that they will definitely be going there until after you have a place allocated. So honestly, it really makes very little difference if you know.
(Also, make sure you actually visit the school. I really hated our local "outstanding" schools and they would not have been right for my child.)