Hey K... well, if you put it like that, I dont blame you for not wanting to go! I agree with Bobbi.. can you just be honest and tell him you would like to go out on a date with him? I guess he doesnt get much time to catch up with his family.. you said this before.. what with working 2 jobs, late nights, seeing the kids (?).. it doesnt leave much time for maintaining other friendships/family stuff, so when he got an invite, perhaps he feels its improtant to go? (who knows what your H regards as important hey!).
I remember you said though that his sister had not at all been supportive since your separation and that you were cross at her for that last year.. so I am not surprised you dont want to go go out with her.. does your H iknow that? Is he aware she was not at all helpful to you when you were S?
Just seems to be a communication breakdown generally, but then, he is not talking to you is he !