I've always gotten along especially well with my hot older sister. Nothing ever off the table for joking around, talking, flirting, etc. A few years ago, I helped her clean a friend's hunting/fishing cabin during the Summertime. She wore bikini bottoms and a cut tank top, as it was humid and only us there.
We're going about our business cleaning the place, and sat in the sunken living room for a break. She wanted to stretch out on the floor to cool off, and I joined her. Within a few minutes, our joking around and hands-on-body play became something else; There was no "we shouldn't" talk or hesitation; We had wild, hot sex on the floor until the sun went in.
By the next day, cleaning the house was secondary to continuing our fun. Living room became both bedrooms and kitchen. I admit, and confess, to nailing the daylights out of my hot older sister for two days in a row. We'd take a break, have sex, take a break, have sex, until too tired to stay awake. Damn, I want to do it again!