I think she is jealous, and probably feeling left out, but choosing to express it in a prickly, snarky way. I think if she just said, "Oh, that sounds like fun.. I'd love to join you guys sometime!" it would be much more productive.
Why am I picturing her as Sara, the petulant dinosaur from Land Before Time?
Anyway, I think maybe next time she said something like that, I'd just cut the BS and address it directly. Not aggressively but kindly.. "SIL, you seem to say that a lot. You are more than welcome to join us any time.. we were planning to go to lunch on X date, would you be able to come?"
Basically, model the behavior that you'd like to see in your interactions with her, and maybe she'll respond in kind.