Don’t trust MIL. How to move forward?

Just need to vent and hopefully get some perspective. Things have gotten really complicated with my MIL, and I’m at a point where I don’t know how to keep navigating this without losing my mind—or losing trust in my own judgment.

Some context: I actually met my MIL before I met my partner. Not closely—we just crossed paths a few times socially, mutual acquaintances, that kind of thing. She’s the one who eventually introduced us. Since we got together, she and my FIL have been very vocal about how much they love me. Constantly calling me “the favorite,” saying they “picked me out,” and going over-the-top in their praise. At first I thought it was kind of sweet. Now? It feels performative at best, manipulative at worst.

In contrast, she treats the other women connected to her sons completely differently. She doesn’t hide her dislike for her youngest son’s partner—calls her withdrawn, says she doesn’t show up enough, makes passive digs about her being “too anxious” or “standoffish.” Every time that couple comes around, MIL and FIL pester them nonstop about how they don’t visit enough. It’s clearly driving them away.

She also complains about her eldest son’s spouse—says she’s lazy, doesn’t contribute financially, has “weird” parenting rules, and is somehow responsible for keeping the family apart. MIL is especially critical of how this woman treats her stepchild compared to her biological child. I’ve had to listen to these rants like I’m part of some inner circle, and honestly? It’s exhausting.

Then my partner and I had a baby. And at first, it was all love, all attention—until we started setting boundaries.
• We said no kissing the baby. She did it anyway. More than once.
• We communicated clearly that I didn’t want anyone present during labor. The moment after delivery—when our newborn had to be taken to the NICU—she showed up and acted like she’d just missed a front-row seat to some big event. She’s even said since then, “I was so close to being there.” That really bothered me. I wasn’t even fully present for the experience myself—it was scary and traumatic. It wasn’t a social gathering.
• We have a private photo-sharing app that doesn’t allow downloads/screenshots. She keeps asking for access to save or share pictures, even after we’ve told her no.

The biggest shift came recently. She and her husband are divorcing, and it came out that she’d been hiding a relationship behind the scenes. That revelation—combined with her repeated boundary-pushing and the gossiping about other women—has made me realize that I no longer trust her.

She’s never kept our child alone for more than a short visit, and I honestly don’t think I could trust her with unsupervised care. Not just because of her behavior, but because we clearly have very different values, especially around safety. She doesn’t seem interested in learning current infant care practices and tends to downplay my concerns as “overprotective.”

Here’s the thing: my partner is in my corner right now. He agrees with our rules, he sees how she pushes limits, and he supports the boundaries we’ve set. I’m incredibly grateful for that. But I’ll admit—I worry about the long-term. I worry that at some point, the pressure from her might wear him down. That she might play the guilt card or manipulate her way into more access, and he might get caught in the middle.

I guess what I’m asking is: how do you prepare for that? How do you strengthen your relationship as a team when there’s a persistent, boundary-testing family member in the background? How do you make sure your partnership stays solid before it becomes an issue?

If you’ve been here—how did you handle it? How do you protect your child and your peace without setting fire to the whole family?