male kittens trying to nurse on step sister

ok, My 4 month old male kittens, siblings, are "nursing" on their 5 month stepsister. What is up with this? I know I live in Tennesee, but this is rediculous. I am not sure if they are regressing or progressing into malehood? The vet says that I should fix all of them in early September.

i know that i have used to 'Yuk' spray when my kittens would nurse off of each other. just make sure that it aint got bad chemicals in it. it took care of the problem.