While it could be extremely tempting to confront them, you need to refuse to make the competition unhealthy.
They shouldnt have had sex in the first place it should not have gotten that far.
Wait for her to say it in terms that cannot possibly be misunderstood.
i am not sure these strange experiences are that only i had? I did try and warn her not to go there but she didnt listen! Some might go to the extent of eating food from the same spoon or fork as yours.