I believe all primary schools give siblings priority unless it's a single gender school and the sibling is of an opposite gender.
From what I know, I do not think that you can apply two. You have to apply one and if not selected then can apply another.
As to distance to school if stay with grandparents, that is an interesting question because they normally refer to the residential address. Unless you are leaving child at grandparents home for weekdays? Can't really help you on this. Sorry! It will be good for me to know too if there's anyone who can answer this :)
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