Sports between siblings can cause jealousy.
My twin boys always do everything together even now when they are in grade 6 but when they just started school things were much worse. Their very first athletics meeting in grade 1 was a day hubby and I will never forget.
Our little babies were suddenly trying to be so big. They had a great time until the one, Kyle, outran his brother Ryan in the race and came first.
Ryan was totally disgusted with his brother and would not talk to him. The first thing he said to me after the event was "Mommy, he didn't wait for me!"