I'm not even a regular on this board (usually on SAHM and dd's birth month board), but I was lurking because it's slow.
Just wanted to say that it is a tough situation...you probably don't want to just let the friendship go, but also don't want to make it awkward, etc.
A very wise friend of mine coined an acronym QTIP meaning Quit Taking It Personally. Unless she has come to you and said you are doing something wrote, don't assume her behavior is about you. You don't know what is going on within their family to make her make the choices she is.
My advice is that if she continues the behavior, then ask yourself, do you still enjoy inviting her on the chance she'll come? If so, keep doing it. If it hurts you too much, then stop and see what happens. I hope it works out though!