So..... the mother has brain damage right? Insignificant duke house? Sure marquis might just be one step lower on the chain compared to duke, but the difference in power is VAST. A duke would be able to squash the marquis household without missing a beat. And it was also stated in like the first 10 chapters FMC's household was the strongest ducal household in their kingdom/empire. In other words, the only household stronger than FMC's household is the emperors.
I hope they fucking obliterate the marquis household, and then the crown prince steps in and make sure they are exiled. I want them to go from riches to rags.