Interesting reponses, some seem contradictory. Do as I say not what I do?

23 years is alot of history. Why would anyone throw it away so easily?

Lay it out for him! I'd tell him to cut out any communications with the other woman, or else. I'd also recommend seeing a marriage counselor together. It doesn't sound like things have gone too far with him and the "sister" yet. But from the sound of it, it will get worse unless theres some sort of ultimatim.