do you think this is fair?

Hey again Justin, I know life really isn't fair but the truth is it isn't your sister that needs you its that guinea pig and by your taking care of its needs you are being a good person. Im not saying your sister is bad, you can't think of it in those terms. Its more like, no matter who this pet "belongs" to it still needs food, water, love and attention, weather your sis appreciates it or not doesn't matter, that pet can't thank you but it sure does depend on you to do whats right, I'm really proud of you for stepping up and taking care of it since it cannot do those things itself, I bet your heart is big enough that you dont need to worry about whos the owner you've already proven your its caregiver. thanks for being so grown up about it, Im certain that pet is so glad you're there for it too.

Is your sister alot younger than you. If so just let her think it is her pet. No harm no foul. If she is the around the same age as you then make her a deal. If she cares for her pet for a month on her own. Then it is hers. If she doesn't it is yours. See if she will agree to those terms. During that month monitor and make sure she is actually taking care of the animal. You don't want to let it starve just to prove a point.

no. tell her it's yours or she needs to take care of it.

Okay Justin, not only does your sister say the guinea pig is hers, you do too. It is stated right in your question. But it does take a lot more to owning a pet than just the mine/hers status. You are truly providing the guinea pig what it needs to survive. without food and water daily it will die. So if you want to be the bigger person, allow her to think it is hers and continue to provide the essential needs to the pet and remind the guinea pig You are the one taking care of it. Any one can be parents, it takes care and love to make a good mommy and daddy. You are providing all you can for the guinea pig, be proud of that.

Correction to my answer below (and apologies to Justin): In the third paragraph I meant to state, "It sounds like you are a good brother..." (but I accidentally wrote "sister"). Sorry.