Mists of Avalon by Marion Zimmer Bradley. I read it while I was in high school too so I was pretty young. I usually am not into Arthurian novels but this one focuses on Morgaine (Morgan le Faye) who is my favorite. On good reads, it says
Ho boy, this was a tasty little treat. But being stranded is one of my favorite tropes. You show me any novel where the main couple is forced together, especially in a cabin during a snowstorm, and I will read it. The stepsibling part could have been off-putting, but the couple did not grow up as siblings (and technically aren’t even siblings at the beginning of the story) so it wasn’t weird, creepy, or incestuous.