It sounds like a good time to pull out the "it's too far in advance for me to arrange to be off work yet" card. Closer to the event, it might be time to look doubtful and tell the Demanding Diva that you can't quite remember what is going on that week/end and you'll have to get back to them after you've checked your calendar at work (if you're home) or at home (if they call while you're at work).
Once the date is firm and the invitations are actually sent (who knows, the site may be booked solid for a month before & after that date) a decision can be made if going is worth the time, money, and effort (or not). Then "oh, something came up at work"; "I'm sick"; or "I don't have the funds after having to fix the (car, plumbing, buy a new washing machine, etc. - whatever they might recognize as a major expense that HAS to be paid - maybe your daughter is graduating college that day and you had to pay for that party - or something).
We were in the military - moving was what happened. The parents came to visit us and we went home to visit them. Siblings - between traveling for work, child visitation for divorces, a spouse who ONLY wanted to travel to see her family after the children were born, and various & sundry other reasons - one sibling did visit several times, the others quit at a certain point, and we would see them only at the respective family gatherings. If they went - sometimes they had to go to the other grandparents' for various reasons or they were home only due to chicken pox or measles in the family...............so they were "home" but not up to visiting or being visited.
There are a couple of family members that I haven't seen in years............because they don't travel at all or because their travel is not in sync with our travel plans. I don't call it "toxic" - I do say that we aren't that close at the moment.................