I read this a long time ago, probably 10-15 years. It centers on a brother and sister who establish pretty firmly that they want each other, but (I think) never do quite get past the idea that it's wrong. There's a lot of groping, flashing and kissing, and I think the farthest it goes is they pick up another woman for a threesome while they're on vacation, and they're basically using her as a proxy for each other.
This one is more recent, with a similar feel to it:
https://storiesonline.net/s/16270/the-wilhelm-scream
