Nephew1 (the one now going to middle school) was registered shortly after birth. Nephew2 (the younger brother), wasn't registered until over age 1. Apparently, because he didn't register "early" enough, Nephew2 had to go to another school for "space" reasons. Yes, siblings get priority, but it's no guarantee. Didn't help that Nephew1's elementary is in the top 5 most requested schools or something like that. Might be in the top 3.
Not all school districts are that bad though. I tried to register DS when he was 1 out here. The school lady official laughed (in a nice way) and told me not to worry about it.