Why would a dog that was once skid-dish become hostile toward the other household dogs after ones death?

My dog, a jack russel, is usually skiddish and playful toward the other dogs which she's know for nearly 6 years is now wanting to jump on the other dogs and start bad fights. about 3 weeks ago my sister's dog past away and now my dog is doing this. my dog and my sister's dog were fairly close and they rarely fought. any oppions about this?