possible/probable mistakes on my part, - I said to W it was nice to see her and i hope she has a nice week. she at best grunted in response - I said 'SIL has sent me a letter which i'd like to talk to you about at some point but its not urgent' she replied by saying she knew SIL was going to but didnt know what she was going to say.
I don't think there's anything wrong with being pleasant to W. In fact, it can be kind of amusing to be nice while watching WAS be grumpy in return. A little game if you will.
But do you really want to talk to W about SIL's letter? What's your goal there?