Want a dog, but can't because of sister? What to do?

Okay, so I was bored a couple days ago and was surfing the web. I came across this dog, and literally fell head over heels. Something just clicked, and I honestly felt like I needed that dog, and vice versa. It’s not even a breed I would normally want! I barely ever ask for pets, because I feel bad. So usually I will tell myself no, and push the thought away. However, I can’t do this with this dog. So I slowly and cautiously approached my mom about how I wanted a dog. She smiled and seemed all happy that I did, which is the exact opposite of what I expected. It was weird. But I went to bed after that, and decided next time I’m with her I would show her the dog. And then, last night I freaked out that someone would buy the dog, so I texted her about it. She said he’s adorable; which is true. Then she asked if it was a Shar-Pei, I said yes, and she never texted me back. So then, later I texted her again and asked if I could buy him. I pointed out that I would seriously take full responsibility, and pay all fees. But she said she knows I would, but what about my little sister? She informed me that my sister is always asking for a dog when she sees one for sale. It wouldn’t be fair. But, really, life isn’t fair. I told my mom we could get her an easy to take care of, cute, little animal, but my mom rejected; reminding me she already had a Guinea Pig. The reason she can’t have any more is because she doesn’t take care of them. So, because my sister is a bad owner, I can’t have a dog either. That’s punishing me for something I had no part in! I just feel like my little sister is always controlling me. It’s annoying. Everyday I get online and look at that puppy, and make sure no one bought him yet. I just honestly feel like I need him. Please help me. What do I do about my sister for my mom to agree? Mother obviously wants me to have the dog and feels bad for me, but still says no because of her.