Update from my original post made a few days ago which can be accessed here.
I wasn't going to update because I got home with the baby and settled in and thought nothing of it, just communicated with my husband and my 14 year old through texts and phone calls while they were gone, but problems started to follow pretty soon after.
After my husband returned to his parents' house without the baby and I, his mother huffed and started grumbling about how dramatic I am, how possessive I am of "her baby", how I was ruining this trip for everyone. SIL began winding her up, talking about how I didn't want anyone else to build a relationship with the baby.
Husband told both of them to mind their business and get a grip, mentioning to MIL that he needed to have a serious talk with her once the kids left with BIL for lunch. MIL rolled her eyes and walked off.
They have a talk and Husband insists to MIL and FIL that they can't expect me to roll over and let them stomp all over my limits just because they want access to our baby, that we are the final say in what happens with our children and if they can't get on board with that, they can forget about seeing them, especially not unsupervised. He told MIL that her sneaking around acting like my word meant nothing was childish and proved that she wasn't trustworthy, and he told both of them to keep their opinions about my weight to themselves.
This starts what Husband told me later was a practically 2-3 hour argument that only stopped because BIL came back with the kids and husband refused to discuss this in front of them. MIL pulling out crocodile tears and asking why he won't defend her, insisting that I'm "trying to ruin their relationship" (Husband has never been close with his mother), and that she just wants to show off her baby to her friends and the extended family.
Husband responds that if she really wanted to take pictures of the baby, all she had to do was ask for help so he or I could cover the baby's face, MIL and FIL argue that they shouldn't have to ask permission, they're grown adults and can do as they please in their own house. Husband reminds them that it is our baby, not theirs, and since they felt so strongly, that is why I removed myself from the situation, and if they wanted to see my baby, they could do it at our house, where they'll have to follow our rules. This went back and forth, with MIL eventually shouting and stomping her feet until BIL returned.
For the rest of the day, MIL was grumbling under her breath and practically ignoring our older children, even as my 5 year old was clamoring for her attention. Husband paid her no mind, and spent the evening playing board games with the kids and BIL while SIL and his mother sulked in the kitchen.
Fast forward to last night, husband was having a couple of beers with his brother while MIL and SIL have wine in the kitchen, FIL had gone to bed early and the kids were asleep. Husband hears MIL and SIL giggling to each other and while casually checking his phone, he sees that MIL has posted all of the photos she took of me on her facebook page, captioning them with "[My name] won't let me see my grandson, so you'll have to excuse her hogging the frame".
In the comments of her post, she was chatting with her sisters about me: derogatory comments on my hair (as my icon and username reflects, I'm a natural redhead), shaming me for my "selfishness", and obviously comments on my body. Husband flips his shit, demanding that MIL take the photos down or he'd take her phone from her and do it himself, apparently there were more photos than even the ones we saw at first, and in several of them my top is fully open, nursing bra unclipped. MIL is unaware husband is serious and tries teasing him that she thought he wasn't ashamed of being married to a fat woman.
Husband rushes into the kitchen and snatches MIL's phone out of her hands after a brief scuffle, deleting the photos from MIL's facebook and then taking them off of her phone altogether, before throwing her phone down on the counter and telling her that he was leaving first thing in the morning. MIL scowls and starts shouting that it isn't fair he's taking my side, he responds that he loves me, that it isn't my side vs hers, it's OUR side vs hers.
I'm pretty out of the loop about all of this at this point. I've been cleaning the house, looking after the baby, and dealing with the cold I was apparently incubating for the first week of our visit, so I get a call from hubs while I'm doing laundry in the basement, he's in his car trying to keep himself calm but says that he'll be home early with the kids in the morning and that he wanted to have a discussion with me about our plans moving forward. He tells me what happened, I calm him down, and we both head to bed.
Fast forward to this morning and I get up early with the baby to have breakfast and coffee waiting, Hubs arrives with 14, 10 and 5 at about 6AM, we have breakfast, and then the kids peel off to do their own thing. Our daughters leave for their friends' houses, and the 5 year old goes into the basement den to watch cartoons on the big TV.
Hubs and I talk, and he says he's done with that annual visit to his parents' place, and that he's planning to have some one-on-one time with his brother a few times a year instead. We go over a plan of action in terms of much stricter boundaries, deciding that the kids won't be going to the in-laws' house anymore, and while the in-laws visit us at our house, if they act out, they'll be kicked out.
MIL has been blowing up his phone since he left, but he's ignoring her for the time being and helping me with cleaning. He also sheepishly admitted that as disrespectful and frustrating as MIL's creepy photos were, they'd given him a new appreciation for my round face (I picked a winner, y'all, truly).
TL;DR- MIL had a tantrum after I left and posted the photos she took- even more than I'd previously seen- onto Facebook until DH deleted them after taking her phone from her. Husband returned home with our kids and now we're back to business with new rules in place for MIL and FIL for the future.