Sorry if this is a mess, I wrote some of it while still reading the book.
I understand that Newland couldn't really stay away from the Countess, since they are related through marriage (after the marriage in book two). I'm not sure that really works since the next time they see each other after his wedding is over a year later (and beyond that, they don't see each other ever again - assumed - after she moves back to Europe).
This isn't what has become the standard for love triangles, one girl and two guys with her having to choose between the two. Instead, Newland is engaged to be married to a girl who is considered perfect by the standards of the day. Then her cousin, a Countess, comes back to New York. She's so different from what the expected young women are in New York society. She's been in Europe for a long time and is separated from her husband (he was abusive). Newland has to choose between the woman he thought he wanted and the woman that makes him think about everything that is wrong with the society that he lives in.
Very interesting view on society Newland comes from. He's never really thought about, but Countess Olenska makes him think about what is considered standard in this society. She wants a divorce but Newland talks her out of it.
I felt a lot more connected to Newland, Ellen and May then I did any of the characters of The House of Mirth but I really didn't like any of them. That seems a little harsh to me, but things would have been so much more simple if the Countess and Newland stayed away from each other. There wouldn't have been a books, either but really I just wanted them to choose - either stay away from each other or fix it!