We had similar with Molly and Mabel, Mabel is Mollys daughter but it started earlier in age than your guys and was only one sided from Molly (mum) she really would growl and try to flatten Mabel, (looked really aggressive) for what appeared to be no reason.

Mabel would run away and then turn to look/stare at her mum still quite poised, things got more heated and we were told Molly was possibly feeling vulnerable that her daughter was trying to take her place in the pack or that Mabel was being a bolshy pushy puppy and Molly was just being a good mum and putting her in her place, the latter was felt more likely.

We were told to support Molly in that when Mabel started to get lairy and too playful we should step in and play with her instead of Molly having to deal with her, we were to praise Molly when she did play nicely with Mabel, we also had to be mindful to give both dogs equal attention.

Advice given was to not interfere once Molly had chastised Mabel but to let them both sort it out, advice given was once Mabel has been chastised by her mum No telling off must take place towards Molly (mum), Just to ignore the behaviour.

The behaviouralist who helped us over this period said once Mabel submits to her mother things will calm down, it appears that by Mabel running away a bit and then turning and stareing at her mother was in effect still her challenging her mum and Molly was not accepting of this.

The day came when Mabel did submit to Molly and from this day on Harmony . Molly still puts her in her place not in the same strong way but Mabel accepts this and she is 14 months now, things have been good with them since about 4/5 months old, things got heated at around when Mabel was approx.. 12 weeks old and it took about 6 weeks to ease up.

Hope this helps a little

Good luck