Wow ! Jennifer you are really a strong girl, even if on the inside you may feel like screaming, your actions are something to be admired. I guess I always thought weddings are for celebrating with friends and family so I do find it a little shocking. Having said that, perhaps because the reconciliation of sorts, was so fresh, maybe that's why they didn't want to invite you guys? Perhaps they thought lets take it slow, ykwim?
Anyway, maybe just focus on having a pleasant relationship with your SIL and go from there? Good luck with this.... I know what it's like to have a dysfunctional family and wanting to have normality when you start your own family.