As I mentioned, I shared a room with my sister, who is three years younger than me, and she would often try sleeping in my bunk. I found this annoying as fuck since I thought I hated her at the time, and would generally shove her out until either I got tired of shoving her or she got tired of falling on the floor. Then I got a bit older, and made a groundbreaking discovery: things, especially girls, rubbing against your crotch feels good. So I would dryhump and, occasionally, finger my sister at night.
We mostly stopped after my mom came to wake us up one morning, found us both sleeping in the same bunk in our underwear, and gave us a vague, ominous talking-to about keeping our clothes on if we were going to sleep together. And then later we moved to a house where we each had our own rooms, which was the end of anything sexual with my sister, except once right after she broke up with her first boyfriend at 14, but we stopped that time before really doing anything.
However, my main point was that it got us over the whole sibling rivalry thing, and we got along much better after that period. So I guess the moral is that incest can be good for relationships if you're both young enough?