To those who absolutely hate this because it isn't 24 and 7 mayhem and murder, or the dramatic and tender love story (a la Manon Lascaut) to end them all - just go on to something else. The whole point of this manga is to be hilarious, silly, ecchi fluff. It definitely succeeds at that. It's broad comedy, and you're supposed to read it for the laughs.
On re-reading this I noticed something. No matter how much of a flirt and how enticing Rosa was as a teenager what Sophia's brother did is called rape. What Leo did, then and later, was to blame her for it. Another point I noticed when Leo was getting all horndog and still thought the FL was his first cousin: the family is traditional Italian and therefore almost certainly Catholic. The Catholic Church does not condone cousin marriage and regards sex between cousins as incest.