I'm having some trouble with this dog. About a month or

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I'm having some trouble with this dog.

About a month or two ago, he started to limp. About three weeks ago, he started to randomly snap at people and other dogs and try to attack them, particularly children.
Before this, he was the sweetest dog I've ever known. I have to believe that whatever is causing his limp is also causing him to be aggressive.

There's a big problem though - he isn't my dog. He belongs to my little sister, who promised she would pay for all the dog's expenses and take care of him. Soon after saying this, she quit her job. She no longer cares for this dog and leaves it at home all day while she hangs out with her friends. My parents, despite saying they "love" this dog, will not take him to the vet to get his limp checked out because they don't want to hear that it's hip dysplasia or something and have to pay a lot of money. I just started college and what little money I make at a pizza place goes to the animals I own myself. My parents make $160k a year and they will not take this dog to the vet.

So my question is, what the fuck do I do? I need this dog to get the care he needs because I love him and don't want them to have to put him down for biting people. I am not responsible for this dog, but he deserves to be treated better. Last time I was home I heard them discussing putting a shock collar on him and making him an outdoor only dog.