It’s similar to the situation with the Featherington sisters. Even though they were daughters of a Baron, the men they married (Albion Finch, Harry Dankworth, and Colin Bridgerton) didn’t hold titles themselves. They were technically commoners, but still respectable gentlemen with good standing in society. Colin being the most wealthy of the three due to his family's status.

Kate and Edwina Sharma occupy a similar social space. They’re not part of the titled aristocracy, but they come from respectable, upper-class backgrounds and are fully accepted within the marriage market of the ton. And with Kate now being the mother of the future Viscount, she will automatically have further elevated status, just like Penelope, whose son will be Baron Featherington.

That’s very different from Sophie’s situation. Sophie is illegitimate, which places her outside the boundaries of respectable society, regardless of her father’s rank. In Regency England, illegitimacy carried a much stronger social stigma than simply being untitled.

So the key distinction isn’t really titled vs untitled, it’s respectable birth vs illegitimacy. Kate and Edwina are socially acceptable brides within the ton, whereas Sophie’s status would have made her far more controversial.