I'm not going to help my sister clean up her cat's mess
Oh for sure that sounds surprising to me. Why don't you want to help her clean up?
Because she's not happy with my decision to go out, and I am not a child anymore.
I get your frustrated that your sister doesn't agree with you, but you should still help out. The cat is yours too, and it is good to be kind to your family and take on challenges together. You can't get this kind of bonding experience elsewhere.
Why do I have to take the blame? My sister should clean the litter up herself.
No one is blaming you. It is just natural for family members to help each other out when needed. That is all you have to do, it is simple.