u/CatCatCat55

I have been searching for a book a read several years ago but can’t remember the title or author!
It was set is Victorian (or so) London. The main things I remember is that the MC woman was an aristocrat but ran a secret opium ring (or maybe is was a different drug?). She had a brother who helped her I think and there was like a secret cellar in her house that she conducted business from. She did not do the drugs but was addicted to sugar and ate a lot of sweets and sugar in her tea. I honestly can’t remember anything about the love interest but maybe he had challenged her to give up sugar? And maybe he was investigating the drug ring and finds out it’s her all along?
I remember loving it and want to read it again! Help!