sure that is the rule of the game. that's why some parents still prefer aided school if they have more than 1 kid. you should also consider:
siblings could be good as well as BAD reference! IF the elder brother or sister is not doing that well at school, why do you expect the school to take his/her sibling?!
once a principal said during a briefing section: we would highly consider our students' siblings, but in case of kids with Down's syndrome, for example, sorry, we can't help.