looking for a story

ive been racking tags and lists for DAYS but i cant find this one .. please help!!

- child protagonist
- royalty (nobility specifically)
- regression (i think)

so theres a specific plot point i remember
- she throws a tantrum to go hang out with her fiancé’s family
- turns out the mom of the fiancé has a crazy cousin who is obsessed with her (mom)
- mc ties a rope to the carriage door and doesnt let the cousin in
- this repairs the marriage of her fiancé’s parents

this sounds insane but i remember the story so clearly pleaseeee helpp （/TДT)/