TBH, how the characters actually react/respond IS the meat & potatoes of most any story – that’s definitely an exercise best left to the writer.
Your first question, when they wake up in the compromising position. If there’s been a mutual pent up secret lust for each other, well, that’ll end fine and dandy. Anything else and you’re talking fireworks, with many twisted outcomes available (more than I can count). For extra flavor, you could have one party *think* its worse than it is/was, leaving the other dumbfounded on the response. Extra spicy is to have a third party setting it up and/or taking advantage of it.
Second question… could use a game of truth or dare for the expose, or similar. For twins, mixups are very common, so having the guy profess his love (or similar) to the wrong twin? As to reactions? Depends on the personalities/trusts/fears. Maybe the sisters get along and decide to share him. Maybe it becomes a love triangle, with the other sneaking in to interject herself? Maybe the sisters have a falling out over this and don’t speak for a dozen years until forced to (ie family funeral).
The better question is where do you want to take the story?