I agree that Logan always irked me for some reason I can't put my finger on... I really liked Heaven and Troy and was so sad when it came out that they were related.
My favorite couple is probably Cathy and Chris, as bad as it is. Every time I read the books (after Flowers and Petals lol) I find myself aww'ing after things they do and then scolding myself and having to remind myself that they're siblings. I liked
Seeds of Yesterday because of the part where Cathy thinks of how she and Chris have forced themselves to stop the things they did as siblings ("My lady Catherine" and such) and stopped thinking of themselves as that way and started seeing each other only in the role of wife/husband. It made it a lot easier to stomach because no matter what started in the attic, I just don't see being able to sleep with my brother thinking of him as my brother... you know?