2 nursing queens 1 spitting

I have 2 cats both had a litter 2 weeks apart, when Pepper had her litter Zoe spat and her and kittens but this stopped after a few days which was great, but Zoe started to steal the kittens at night so we seperated the cats at night or when we werent there. Since Zoe has given birth Pepper has continued to spit at her and her kittens though its not all the time. They eat together and i encourage play together at least once a day.  Any advice or help would be great, thanks 

Do you have practical possibility to have them separated?  In each one room?

This is perhaps the best.

They dont seem really hostile to each other, yet they feel they must mark "beware, dont dare to touch me, or else!" - having their defences out.

Perhaps not so peculiar. Moms  often get into extra protective mode.

One classical advice when they are non friendly is to have different feeding places, sand-litters, resting places, etc, so they dont need to confront each other. The only forced meeting which remains is when they occasionally want to play with each other.  

I mean, I understand you want to socialize them with each others. In normal cases it is good and well, but perhaps not at this moment...

If you CANT separate them, or by any other reason dont want, make sure to down grade all stressing moments you have influence over.

You can also raise the relaxing by having a Feliway diffuser on in their room. Playing calm relaxing classical music - harp music is best...

There are surely other possible ideas.

Good luck!