There could be several factors going on which might be impacting on her control...
Some could be caused by hormone changes, as she approaches teenage years and the transformation into a women..
Her levels could be reacting to stress at school, secondary school will be a lot different environment than her smaller primary school..
She could be fipping about what she actually eating or and the insulin she's taking.. This could stem from how she percieves her peer's reaction to her diabetic condition..
I think it would be wise to contact her diabetic care team, so that they can look at her results, and if there is an underlying problem that your sister might need some extra help dealing with along side help with improving her control..