Out of Spite?
Here’s another poo-related story that’s funnier than it is gross. This kid’s mom caught him trying to take a dump in the trunk of his sister’s Barbie car for reasons. Perhaps it was out of spite. You know how brothers and sisters don’t get along, at least for the first 10 to 20 years of their lives.
The silver lining, if you want to think of it that way, is that the OP and his sister (and probably the mom, too) have a pretty funny story to share but not around the dinner table. They’re more than free to share it with strangers online who’ll laugh at his failed attempt to ruin his sister’s toy car.