How would we get alone?

ok, i really like like a one of best Friends little sister. first of, dose anyone think its weird that a highs schooler going out with a 7th grader weird? she is exactly one year younger then me though. i know this because i was at her birthday. all that im wondering is how in the hell would we be able to get alone together.the only time im at her house is when im hanging out with my friend. l:( i really cant ask out his sister in front of hir. just wondering if this might be usefull, but we hang out at the mall sometimes. just need sum ideas on how to get alone with her to ask her out????? TY