Genetics...

Are a funny thing...  

These girls are 3 1/2 years apart...

 And these girls are just over 2 years apart...

Brooke is the 2nd tallest kid in third grade and while lining up for the Christmas concert, the twins realized there are only 7 kids shorter (or the same height) as them in all of first grade...  Which makes them look a lot younger than Brooke.  And Sam is average height (while being barely on the scale for weight) which makes them look closer in age.  In another year or two, I'll have triplets...