I made a joke about my sister pooping her pants.
For what reason would you joke about that?
I was a little fed up with her farting.
While it is okay to joke around with your siblings, taking it too far can not only be rude but also hurtful and embarrassing. You shouldn't claim that your sister defecated in her pants when it didn't happen.
It was harmless. It's a bit funny, don't you think?
So long as it was harmless and your sister wasn't hurt by it, it is a silly joke. Just make sure that you don't take joking around with your sister too far as you don't want to hurt her feelings or make her feel uncomfortable in front of others.