How to stop my cat's bullying behaviour?

[Flossyfloof]

I have two cats very close in age to each other. They are actually from the same mum, but different litters. They generally get on well, although the older one, the boy, sometimes has a go at the younger girl. They often snuggle up together and wash each other. Some time after I got the kittens, I took in their mum, so although we all live together they were separated and the younger ones don't recognise her as their mum.
The mum cat adores me and is quite a one-woman cat, really. The other two ar a bit scared of her. She especially bullies poor Ruby. For example, if mum is on the bed( my bed I mean, obviously) and Ruby walks into the bedroom, mum sometimes gets really nasty, growls and hissy. It is really upsetting. Right now, mum is sitting next to me, Ruby is sitting being really good, looking out of the window. I can tell her mum is giving her the evils and would chase her away if she attempted to get anywhere near me. I feel that poor old Ruby is scared to do anything.
Any advice please? This is a long-standing thing, I haven't just got them. Maybe I should have done something before. I was wondering if it might be worth getting some Feliway? I am especially worried as I am having some building work done soon and that will be stressful enough for them all without the bullying.