JaCustomer,

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I'm surprised that your puppy school has never seen this before as it is very common in households with more than one female. About the only way it is a little different is that usually the pups work out who is the alpha female by this time without a lot of problems. However, if the dogs are evenly matched, you do have problems like you are seeing.

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Dogs do experience hormonal changes as they go into heat. We do normally see some aggression toward male dogs at the beginning of the heat cycle since they are not yet at the right time of heat. We also sometimes see aggressions toward other females to establish their leadership and "right" to mate. If they are not spayed, then it is about th time they would go into heat, so they would be more aggressive toward each other. If they are spayed, then it is just that they are maturing into adults and trying to determine which dog is going to be the alpha female.

I would discourage any aggression by your female at all times. There are other causes for sudden aggression in dogs such as hypothyroidism. You can read about this here.

http://www.petplace.com/dogs/medical-causes-of-aggression-in-dogs/page1.aspx

When a dog is a puppy (under a year of age or so) any older dog in the house is the alpha dog. A male is usually alpha over females and females have one female alpha in addition to an alpha male. Thus there is no dominance fighting from the male. In two dogs that are evenly matched in size such as sisters, the one that is more strong willed and dominate will likely be alpha. If both are evenly matched in that respect as well, you can have issues until their is one clearly alpha females. Sometimes it will look like one dog started a fight, but with dogs a wrong look or movement by a submissive dog can trigger a fight.

I would start making your dogs work via the Nothing in life is free program (NILF). It is outlined below.

http://www.pets.ca/articles/article-dog_nilf.htm

You will also want to keep a leash on the females at all times initially to grab if they should disobey. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.

You are on the right track with obedience training. Definitely keep up with obedience training as this can take the alpha position away from both of them and put it where it belongs, with you. If you are the alpha, they have no need to fight. Usually dogs will eventually determine which one is the stronger more dominant dog and the fighting will stop, but before that happens you can have some vicious fights and potential injuries so it is best to stop it now if you can.

In addition, if the situation is not improving using the techniques on the previous website, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.

http://www.apdt.com

Living in a household with multiple females can be an issue. You may eventually need to keep the two separate to avoid these problems. You can read about this issue here:

http://www.canismajor.com/dog/feisty.html

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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.

http://www.fda.gov/AnimalVeterinary/SafetyHealth/RecallsWithdrawals/