During my first marriage I had the opportunity to make out with a really cute and naughty sister in law which led to a few times of oral sex, and eventually I got to fuck her once, yes it was wrong, but damn it was exciting and good!!!
Spoke with a couple buddies over the years about such a venture. According to their findings, there was very little to zero difference in the fuck experience with the sister in law versus the wife. It makes sense, though.