now my mum has read that if you lie a dog on their back and gently push down on their ribs and growl your showin them whos boss but all bailey does it growl back at her more louder and shes gettin aggresiveArghhhhhh.......

(holding shaking head in hands) No, No, No.
I agree with others this is outdated and they are but pups, what you are being advised to do is wrong and harmful, this is a
12 week old pup who is doing what all pups do, pups
do play aggressively, they play fight all the time it is how they learn, they pull bite, drag and growl and adult dogs know this, which is why they are tolerant to pups behaviour, hierarchy status, dominance and pecking orders do not even come into it at this young age it is all about play and learning.
Your 10 monther is not even an adult herself, you have two pups here so she is being very good about her younger family member, you should not interfere only to give your 10 monther some alone time if pup gets too much.
You have two bitches with a not perfect, (as ideally an older bitch would have been even better) but at least with enough of an age gap to hopefully not need to worry about making an alpha yourself here, the older one should naturally be the alpha, if not there may be problems a good few months ahead from now, and if so your techniques of feeding, grooming first a chosen Alpha
may work then but with Staffies of the same size, weight and character it may well not! That we shall have to wait and see in the future and hope that the once fully grown pup you have is not equal to your older bitch in mind and body, if she is and both wish to be alpha's you will not be able to control the outcome with this breed.
This is why a good age gap is needed to avoid these issues from mostly happening.
In the meantime, leave them to it, your older bitch will let your younger know in a few months time that playtime is well and truely over, the pup will either have a big sister to look up to (and I have my fingers crossed that this will be so :-) ) or will try to become lead bitch herself.