Help me find this book - young adult US based

[mouche202]

Hi. I am trying to find a book I read in high school but have forgotten most of the details. Was hoping the hive mind of Mumsnet could help me track it down.

What I can remember:

• I read it in about 1996
• It was a teenage romance set in the US
• Plot was from teenage girl's point of view. Parents divorced, dad brings new girlfriend home who has son same-ish age as the main character. Dad wants kids to get along. Daughter hates the idea of step brother. The kids end up going on some kind of trip alone together. I think it was on the river and may have been an accident rather than intentional trip. Bit fuzzy on those details. Anyway they fall in love, have sex and want to get married. Come home, tell parents who are horrified because they are actually half-siblings. Boy's mother was OW. I forget exactly how it ends but that's most of it
• I think the title has Moon in it
• The boy's name is probably Stephen

Any ideas welcome!

Hi OP. I thought at first you might be talking about this book from 1985:
en.wikipedia.org/wiki/Family_Secrets_(novel)
which we had but I somehow never got around to reading, but the boy's name is not Stephen, there is no half-siblings plot and it was the girl's mother who was the OW.
I also found a book called Movements of the Moon: a Stepbrother Romance by Celia Styles. I couldn't find a publication date and, with there being only one review on Goodreads, the details were somewhat scant. However, it seems to be classed as an adult erotic novel, and yours was YA, so I don't think that's it, either.

@ComeDoonTheStairs, what a detective you are! Family Secrets does sound very similar though there's a lot more to it than the one I remember - I think I will get and read a copy to check if it is the same one. Maybe the discrepancies are due to faulty memories.

@Mouche202
I hope you enjoy Family Secrets. If it ended up being another book I'd be very interested to know... the half-siblings plot sounds very interesting and I hope it was true!