You don't need to say anything to sil about who is leaving who it will be glaringly obvious to all in time.
If your h doesn't snap out of it an continues with his Mlc he will take vacations without his kids, date a younger woman, dress differently. My h family was confused at first, cause my h told them it was mutual but it became so obvious and I didn't say a word
---- M 39 H 35 D5,D4 M 4 T 9 ILYBNILWY 5/18/11 Left 7/11/11 Divorced 12/1/13