Okay, Jen. Hopefully we've been working together long enough that you can handle a little "tough love."
(And I'm taking it that you were invited to go to dinner, but decided to stay home--let me know if that's not the case.)
Since they have invited you, but you've turned down their invitations, you're helping to keep the bad feelings alive. If you want to re-establish contact with the rest of the family, you're going to have to interact with them again. It will be really hard, and you'll probably have to "fake it 'till you make it" to be friendly with them again.
Start slow with short exposures to them..just an hour or two at a time. That way, when bigger occasions come up, it won't feel so weird being around them.