Wife's Sister

I have fancied my wife's sister for years but never thought I would get the chance to ever fuck her but an opportunity arose a few weeks ago and we had amazing sex. When we finished she said she didn't want my cock to be used in her sister ever again and to make sure she wanted it locked. I agreed and bought a tiny chastity cage which she fitted on me and kept the key saying my cock was now hers and not my wife's. Whenever she needs servicing now she calls and I have to go round and she lets it out so I can fuck her. Before I leave she locks it back up. It's been a good arrangement for us both as my wife and I have not had sex for a long time and she doesn't even know that I am in chastity. I have found that being locked and not in control makes the sex so much more exillerating and I never want to be free of this device again.