How can I convince my parents to let me and my sister get a mouse?

OKay, so I already have a hamster that I take great care of. I also have 3 cats and a dog. Me and my little sister (4 years old) want to get a mouse, and at PETCO they are only $3! We already have a brand new cage, the crittertrail treat zone habitat, that we won at the PETCO hamster ball derby. We also have 5 extra bags of hamster/ mouse food and 3 extra hideouts, an extra wheel, and 2 extra water bottles. I see no reason for my mom to say no, but she is persistent and won't give up. I already have close to $100 and I could easily pay for the mouse and all of it's needs. My dad is fine with it and could care less, but my mom is stuck on "no" and will not budge. What should I do?