Agressive dogs?/ HELP

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So in our household we have 3 dogs. dog #1 is female, and she is half jack russel, half australian sheperd, now she is the good dog in the house hold, she listens, behaves, and tends to just keep to herself dog #2 is male, and he is a germen sheperd, and he is good for the most part and can be a pin but does listen, now Dog #3 is also male, and is a mix of 3 different breeds, which i believe are border collie, labrador and austrailian sheperd. We never really had much of a problem having all 3 dogs in the house, the 2 males used to play all the time and got along fine, and as i said the female keeps to herself and on occasion will play. Then suddenly, when the two male dogs meet inside the house they become very agressive towards each. Now my bf and I own the one dog, and his sister owns the other, and frankly i dont agree with how she treats her dog, but she refuses to let anyone tell her how to treat her dog. So the dog is not fed as often as he should be considering he is only about a year old. The second her dog ( dog #3) sees our dog, he begins to bear his teeth and growl and bark, and our dog being protective fights back. We've had a few times where there has actually been blood shed, and am really at a loss at what to do. Our dogs show no agressivness at all, and our male usually just ignores her dog when he barks at him outside. We want them to be able to get along, cause in the summer we all like to be outside, but it seems adult presence in the room only helps initiate a fight. We are planning on getting muzzles for both of them and basically out them in the same room. I do not know if this will work or not, but atleast we will be able to put them in the same room without them being able to hurt eachother. Does anyone have any suggestions as to what we can try to do to fix this problem? could it also be that neither of the male dogs are fixed? anything would help, sorry its sooo long, thanks