Litter box and Kittens

Any suggestions on how to keep the kittens out of the mothers box and to use their own?
Sunshine absolutely does not want to share, so she has her own litter box in the basement. The kittens box is upstairs, but now they are roaming the house and want to use her box as well as theirs! How do I train them which one is for them and which one is for mommy???
The last time Sunshine was mad about sharing, she resorted to using the bathroom sink instead of the box and caused quite a commotion in the house w/ my husband!!!!!!!!!!!!!!!! I don't want any repeats.

Daniela...honestly you cant teach them to not use a particular box. In fact kittens wont learn when thier just beginning unless they follow moms lead. The only way you can get around this is to deny the kittens access to moms box. Maybe there's a way to block it so that mom can jump a barrier or put it where mom can jump to and the kittens cant.

Thanks Sandie, I kind of figured that is what you would say. For now they can't get to it because its in the basement so when Sunshine goes down to use it I just shut the door for a few minutes. I guess she'll have to learn to share when they can get to it!! Thanks for the response.

Sunshine might not react the same way to her own kittens sharing her box as she does when a strange cat uses it. At least there is the hope that it won't be so bad for her this time.