How to stop new ferret from nipping older one

Hello!  I have had single ferrets for awhile and last time I picked up some food for my 4 year old girl I fell in love with a totally black little girl who is just 4 months old.  So now we have 2!!  I've trained Donella not to nip my fingers but she's a bit rough on Mercedes who seems to be afraid of her.  Any ideas on how to stop the baby from nipping her older sister?  I can't use the normal finger training which worked pretty fast.
Thanks!

Put some bitter apple/vinegar on the area where she is biting the older one.

Thanks!  I wasn't sure if I could put anything on my older ferret...she's getting bitten pretty much on her neck.  I was hoping a sister would help her but it seems to be adding more stress to her life.

When you see the little one bit the older one.  You can scruff and drag (gently) little one on the floor and hiss at her.  Every time she goes for the older one....scruff and drag/hiss.

She will get tired of you doing this to her....she might stop biting the older one.

Good luck