2 shit-tzu

Question
I have a 10 month old male shih-tzu and i live with my mother in law she got a male shih-tzu a month after i got mine,hers is 11 months every thing was ok but now they are really aggressive with each other my dog is a little heaver than her dog so he circles her dog and they fight and wont stop she wants me to get rid of my dog but thats not going to happen what can i do and why are they fighting? we have 2 other dogs a male and female Chihuahua and they don't fight with them or vise versa  

Answer
Get both dogs altered...spayed and neteed adn teh problem should be fixed, f not be suer both dogs are getting enough atention from everyone if one is being favored they will be jelus and might be whats sparking fights aslo make sure they each have there own food bowls food fighting are common in any dog, make sure ach dog has there own persoanl space like for my dogs it is there crates be sure each dog has its own persoanl toy only it can play with, dont give one dog a treat without treating the other it can spark fights or jeluisy but most of all get them altered.