the tail wagging the dog
This means that the least important part of a situation has too much influence over the most important part.
Bob:Honey, let's buy new purple carpet to match the purple lamp shade.
Wife:This sounds like the tail wagging the dog.
Wife:This sounds like the tail wagging the dog.
Tail wagging the dog
When a person allows their decisions to be dominated by someone who is clearly unfit for the role, rather than being responsible and taking control of the situation.
When parents let their children make the rules, instead of the other way around, is a prime example of the tail wagging the dog.