lost book with a focus on language

This is probably such a long shot because I really don't remember much, but I'd like to read the book again. Anyway, so it's narrated in first person with someone who is VERY smart and continually not only uses unusual words/language, but focuses on it themselves as they narrate. (He describes things that are bad things he's done, I think, and likes to pretty it up with his language, maybe?) Pretty sure it's a guy, might be unreliable. I read it in college when I was a senior, so I think that is 2003? Pretty sure the book was fairly new when I read it (I think I borrowed it from the college library [Emerson College in Boston]). It was a novel. There are a couple other characters, a brother and sister, I think, and some sort of crime(?) or mishap. I also think the narrator was some kind of criminal himself, maybe? Oh, I hope this rings a bell with someone. I know it's super vague but I think about this book every once in a while and cry a little because I can't remember the title or author or anything more specific about it. I wish I could remember more. (I actually have this vague picture in my brain of a scene towards the end of the book that takes place in an attic, I think. And the brother/sister characters might be outside, being watched by the narrator. It's also possible the narrator was either dating, had dated, or was married to the sister character. And I think maybe some of the plot had to do with him being spurned by her, but I'm not sure.)

Thank you!
This is a stretch, but just in case, have a look at "The Secret History" by Donna Tartt.