She was part of the aristocracy due to her stepmother raising her as a lady. Sienna was a working woman, and an opera singer. Sienna was a mistress already- she would never have been accepted as his wife.

It’s not that Kate is a commoner, it’s more her class within that framework. She’s part of the gentry, opera singers are not

Different kinds of commoners

In the book Kate actually debuts with Edwina and they are both young debutantes on the marriage mart in the season in London completely eligible to be proposed to by even the Duke. They’re not the same.

Because opera singers (and actresses) weren’t just commoners.They were considered, especially by The Ton, barely a step above a prostitute.

Anthony was the first born son and was to inherent the title. His wife would take on his title as well. His wife would have to help him maintain his station. His younger brother does not inherit the title or responsibilities and was freer to marry who he wanted

Kate was raised as a lady. Sienna wasn’t. She was in a trade. She had a profession and makes her own money (through various ways) Benedict could also never marry Madame Delacroix for the same reasons. Also, Kate was assumed to be a virgin and “raised well” and from “a good family” and connected to society. Sienna was part of the demimonde - the society on the fringes of high society (the ton) - a society of courtesans and mistresses of wealthy men.

Kate came from money though right?

I don’t think Kate WAS a commoner. In the books, her dad was an officer stationed in India. Officers at that time were gentry (but usu not titled. So an earl’s brother, not the earl). I know they changed it a bit on the show, but he was high enough in the diplomatic ranks to rub elbows with rajahs. And Mary was the diamond of her season, so she was ton. Also, performers were considered lower than working class. Women in the field were frequently mistresses or otherwise “for hire.”

ignoring everyone else’s reasons, Opera Singers and other performers were often (forced) to be  prostitutes as well, which made her tainted ot somethinglike that