Celesta ... seriously. Either you are dumb as dumb can be or you're 3 chapters behind the script
Well, TBH, her whole family is disfunctional, but atleast it is understandable how it got there. Father was heroing around when he got in love by a beautiful 3rd ( maybe ? wouldn't be surprised ) daughter of a lowly hickland noble family ( that most likely was being prepared to be a trophy wife of some merchant to pay debts, so not much needed in the smarts section ) and marries her against the will of pretty much everyone. As the first daughter was showing no signs of being as talented magicwise as the father, he was pressured ( the context implies he was
royally pressured
) to marry someone of a suitably noble ( and talented magicwise ) family to ensure that the magic talent was not lost. And obviously that the daughter of a great house would not sit idly and be the second wifey, especially when the first wife is ...well, Celesta :/
So, father was forced into the situation by politics ( and most likely is far better prepared to slay dragons than to make budgets ), mother is as dumb blonde a person can be, but had no actual chance of being better, stepmother, while being a evil schemer, was pretty much forced to assume that role because of Celesta dumbness and chibi stepsister is just trying too hard to be daddy's little girl ( the other stepsister so far has been decoration ). And , OFC, Paril was just someone that simply couldn't fill the shoes that were prepared for her by genetics bad luck ( well, not sure if bad luck, because Celesta never shown any sign of being talented on magic terms ) and that was not prepared enough to deal with Esmeralda.
All in all, a nice sitcom family