I have a similar problem, but different in that it is my oldest unrelated bitch that gets in between the mum and daughter

;) I keep an eye on them and if it starts to get out of hand I intervene. Thankfully my youngest is now 8 months old so is bigger and more able to take care of herself and isn't getting too out of hand. In fact she has been put in her place by all the other dogs at one point or another so has learnt how to behave. You little one is still a puppy, but wants to be grown up so will push a bit more if allowed. The other dogs are starting to see her less and less as a puppy, so will put her in her place more. Mum especially will do this. After all, it's a mums job, isn't it ? :) If you think her behaviour is too much then I would intervene as you are doing.