Permission to Cheat: An Interracial Cuckold Tale

It's late and Daniel is about to make the moves on his wife Tanya when the doorbell rings. His sister, Amber, shows up crying because of boyfriend problems. Always the good brother, he puts his own evening on hold to help her out. As soon as the topic gets raunchy, he makes himself sparse but not without overhearing that the argument was about anal sex. So when Tanya tells him later that Amber asked for her help, he's happy to let her, as long as he doesn't hear any of the details.
Except Tanya's behavior doesn't seem to match up with the facts. She is acting oddly but his suspicions aren't confirmed until several days later, when it's already far too late change anything, that he unknowingly gave her permission for something he definitely did not want her to do. He finds out in the worst possible way. Does he have it in him to save his marriage?
A raunchy, 7.5k words short story.