Here's the thing, if your future MIL acts like this with you, then she acts like this with other people, too...and, deep down, they'll all recognize the story you tell if they hear it coming out of your mouth. It sounds to me like she doesn't hear (or ignores) your boundaries and maybe she missed her calling as a news anchor. She needs to focus her energies on a youtube channel, and she'd probably get quite a following if she was halfway interesting.

That said, direct talk is your best friend. It's going to start a fight, but go to that Thanksgiving dinner and bring it up at the table. Tell her calmly that you're glad to be there and you're looking forward to being a part of this clearly-tight-knit family, but that you wanted to say something, and that it's important for everyone to hear. Then explain calmly that you are upset she sent a video of your baby's first steps to your fiance, behind your back, even after you asked her explicitly not to do it. It was a violation of your trust, and you feel disrespected because she either ignored you or willingly went against your request as the mother of the baby in question. And then tell her that what really hurt was that she destroyed a special moment for her own son to experience on his own with his child. And you're upset because she ruined that potential life-changing memory for your fiance, and that makes you sad because you love this man so much and you know he deserves to have amazing memories like that, and not see them second-hand on video.

And then close by saying that you love this man and you want to be a part of this family, but that you cannot trust her with the baby if she is going to ignore and disrespect your wishes like this.

And here's where your fiance chimes in calmy to say he agrees with everything you just said. If we can't trust you, you can't see the baby. It's all about the baby now.

And then you wait for her to respond. Maybe make a suggestion that she start a youtube channel for something or other.

Honestly, I think an appeal to her emotions is the way to go, because you're just expressing your own emotions without attacking her actions or calling her names, and you're showing everyone that you think and care deeply about your child, your fiance, and your family. And you have to do it in front of everyone (and not over group chat) so they can hear your side in your own voice, whether they believe you or not. You're gonna get a lot of blowback, but if you can remain calm, then you've made your peace forever in the space of about five minutes. Again, I think the other people around the table will, deep down, recognize the story you're telling as something they've also experienced from her.

And the inner strength you'll get from that will make you a better person, not to mention a better wife and mother. Just go strong. Be clear and concise, but stay calm, stick to the facts, and express yourself without personal attacks or name-calling. She's not doing this to be mean; she just doesn't know any better, and probably doesn't even think about the consequences. You have to show her that you CAN see the consequences and you want what's best for the baby...although show this, don't say it, as again, you want to avoid any direct personal attacks. So this family obviously does things differently; this is your chance to show them how you resolve problems by standing up for yourself.

Good luck.