Why does Jesse's older sister tease him about his running in Bridge to Terabithia?
Jesse's older sisters tease him about his running because he comes back all sweaty and smelly, and they find that offensive. Basically, though, Ellie and Brenda are always fussing about something; if they weren't getting on Jesse's back about his running, they would assuredly be complaining about something else.
Ellie and Brenda are spoiled and disagreeable girls. Their conversation is made up almost solely of whining and repining. When Jess comes back from running in Chapter 1, they are at breakfast, and Ellie greets him with the sarcastic comment,
"We-ell, look at the big O-lympic star...sweating like a knock-kneed mule."
Brenda chimes in, pinching her nose "with her pinky crooked delicately,
'Oooo, Momma, he stinks...make him wash.'"
She then continues her commentary by whining about the breakfast her mother is making, and after reiterating her demand that Jesse wash, begins complaining with her sister when Momma declares that there are chores to do. Ellie and Brenda wheedle their way out of helping out, securing Momma's reluctant permission to go shopping with friends instead. As is usual, Jesse is left to do the chores for his mother himself (Chapter 1).