In laws have no idea what's happening. My W had only told her dad that we were separating, didn't explain anything behind it. He assumed the worst, that I had cheated or something. She spoke to him yesterday, and he actually told her to tell me "Hi." The shock of a break in a marriage makes everyone cling to their kin, and though some in laws are strong enough to look outside the box, most aren't.
So take the kindness from your SIL for what it is. Be aware though, (and it appears this might be the case) that your H is using your SIL to shirk his responsibilities.
