You did it! We figured this one out! It is a truth universally acknowledged (by me for certain) that the Bitchery pretty much knows everything, and really, it's true. Scroll down to see the solution for this HaBO - and many thanks!

This HaBO request is from ZoeQ, who is looking for a book that may have possibly been reviewed:
Looking for book that was reviewed once before, but couldn’t find it in the reviews.
Set in the 1950’s parents go out of the country (or on an extended vacation) & the sister gets sent home from school. She has to stay with her older step brother (apparently young women couldn’t stay alone in the 50’s). That’s where the trouble starts.
I remembered it when a co-worker & I were discussing book reviews. Now I can’t find it it’s driving me crazy.
This doesn’t seem familiar to me as having been on the site, but my brain is scrambled with upcoming travel to-do lists. Does anyone recognize this?

I don’t remember the timing, but Princess Daisy had this – although he might have been her half-brother. It’s probably 30 years since I read it. Yes, I was far too young to be reading Judith Krantz.
It definitely wasn’t Princess Daisy. She spent the summer with her father’s mistress and half-brother after her father’s death. God, I loved Judith Krantz!
I have no idea what it is, but it reminds me of a story my girlfriends and I read in one of those confession magazines titled We Took Love Lessons From Our Parents. That was more than forty years ago, but I still remember it so clearly.
No mistress the parents were married
I know there is an old Sara Craven harlequin with this theme but I think its 70s or early 80s
I also loved Judith Krantz when I was younger. Using my friend google I found this:
http://www.goodreads.com/book/show/28182571-stepbrother-s-rules
It seems to tick all the marks. Also, there seem to be several other books with the same title.
Thanks Chinami that is the one !!