Not something that they talk about much, being male, but men do react to things in a totally different way to women. Sometimes, when things are going a bit wrong in their lives, losing family members, wives being pregnant [even when they want the child themselves] they use affairs as a sort of escape. I'm not putting forward excuses, but when my husband had an affair and our marriage ended he was totally irrational about the whole thing and didn't understand why he was hurting both me and our children. It was as if he was on drugs. I wonder if your SIL actually understands why he has behaved in such a way? I do understand how you feel about him, though. You can't just stop loving someone who has been part of your life for a long time, but that love depends totally on how they treat your own child and grandchild. Complicated, life, is it not? But you will sit down as a family at Christmas, and time will heal and everyone here understands your sadness. Take care.