Congrats! It’s perfectly great to give yourselves a mutual pat on the back at this juncture.
But beware. The blowback is yet to come. When manipulative people feel exposed in their behaviour, and a bit backed into a corner (especially when multiple family members back the accusations) they tend to come out fighting. Fighting dirty.
It would be lovely if these situations worked that the unreasonable person took some time to do the necessary inner work, realised that they’d hurt themselves and others, and reacted by apologising and setting healthier goals for future behaviour. I’ve personally never seen that happen, ever.
You know the phrase ‘things will get worse before they get better’? I strongly suspect you’re about to see it in live technicolour. But don’t despair. Get in early by continuing your research, on setting and policing boundaries. Agree with each other which boundaries you will communicate to your MIL (& FIL).
I found that quite restricted boundaries off the bat are best. You can always loosen them (carefully & deliberately) down the track if she sticks to good behaviour.
Another tip is: don’t worry if your boundaries sound a bit silly. Maybe you (& DP) decide that every time she makes a ‘dig’ in conversation, you start a countdown out loud (1,2,3). Once you get to three, the visit ends. Not a dramatic storm out, just a smiley but decisive end. Get up, say goodbye, and leave. Yes, you do it even if it’s Xmas Day. She’ll be counting on you to feel too silly to do it. Show her she’s incorrect. Every single time. Ignore any reactions, including from ‘flying monkeys’. Stay firm, resolute and don’t be tempted to respond. Answer only ‘we told you the boundary and what we’d do if it was stepped over. You/She stepped over it, we did what we said we’d do.’ Like a broken record.
She’ll push back like an increasingly irate toddler. I’ve seen people literally turn purple and spit venom. Like with toddlers, don’t react. Sigh, and remove yourself until they’ve figured out it doesn’t work.
Oh, and be ‘too busy’ if she invites you somewhere (or to something) expensive or remote. I got stuck once with no car, in the middle of nowhere. I’d confirmed my boundary by ‘leaving’ but I had trouble getting far! Another time my toxic relative waited till the middle of a concert to make their move- it was really annoying to leave that time, with what the tix were worth!
Good luck. I can confirm it’s worth it down the track. I find I only need to reconfirm the boundaries every 2 years or so these days, and it doesn’t lead to drama any more. A bit like the way a mum can give her misbehaving child ‘the look’ in the supermarket, because they’ve done all the real work over time at home beforehand.
I won’t lie, this next phase will likely be tough for you both. It’s a process. Celebrate each mini-win, and stay ahead of her games by researching, agreeing boundaries (& consequences) and having each other’s backs. She’ll take advantage of any loophole. Good luck!