I have four siblings, one brother is deceased. I have an older brother and older sister. I was the baby by twelve years and I am sixteen years younger than my sister.
I have 4 step-siblings; 2 boys and 2 girls. They are all 3 months to 6 years younger than me. The oldest is a December baby so we were never in the same grade.
I have a little sister a year younger than I am. In Year 3 (the equivalent of 3rd grade), she skipped a year and went straight to Year 5 -- they introduced a short-lived aptitude test that allowed kids to skip Year 4, but was abolished after 4 years because the competition was getting unhealthy. So starting from Year 5, both of us were in the same class year every year, which was odd to say the least.