From your description you have a clear cut alpha and that is Biscuit, but she needs some serious and both mental and physical exercise as she has probably inherited the stronger characteristics of the shepherd
, right now she is trying to wear off her excess energy in rough horse play and that is what is causing a lot of the trouble She is also at the age where you can compare her to an unruly teenager so needs some discipline and structure to help her become a good pet.
You have siblings, who are both females and spayed. Just about the worst combination you can have, and the fierce fighting for domination is going to get worse.
I am going to disagree strongly with this comment, a lot of people I know have same sex siblings and no problems, I know of people who have adopted 3 and 4 littermates and are not dealing with fighting.
I have had greyhounds for a few years now and some can be competitive and will play rough, and during that rough play if one gets hurt it can cuase a fight to occur, so when I have a competitive greyhounds here I muzzle during outdoor playtimes, and all dogs have to be muzzle, because leaving one defenseless can be fatal. The type of muzzle I would reccomend is a basket muzzle it allows dogs to drink, eat easily and does not interfere with panting, so can be kept on for extended periods of time, so can work well in your home both indoors and out. http://www.morrco.com/itbasmuzsizc.html
Your also need to do some training, Nothing in Life is Free is a good training program you can do at home http://www.greyhoundlist.org/nothing_is_free.htm
She also needs to wear off that excess energy with a long walks and running eg. playing fetch, solo without the other dog so no competition is occuring and possibly enroll her in obedience and even agility classes to her work her mind and body. There is a saying a tired dog is a good dog.
Biscuit has also become much more aggressive with our children (11,7,5). She has never growled at them, but she has taken to biting on them, pulling at their clothes, chasing them when they are playing in the back yard. I am very worried that it may progress into something more.
This is not aggression, but a dog thinking of the children as littermates and she is playing nipping with them, but she can get carried away in her excitement and unintentially hurt them so it needs to be stopped. Buy a super soaker and supervise them outside, when she goes to chase the children correct her with a good blast of water from the super soaker and tell her no chasing each and every time she goes to shase. She will quickly catch on to leave the children alone. There is another method which can be used stop the play nipping but it requires the children to stop running each and every time and then stand there with their backs to her and ignore her each and every time she chases them, but the 5 year old may have trouble doing this which is why I recommended the super soaker method instead.
With good training, sufficient exercise and maturity she will become the ideal pet, but work and time is needed to help her reach that stage, if you are unable, not willing or don't have the time then you are better off rehoming her before something awful happens.