My guess here is that your husband is plagued by insecurity and fears harsh judgment by your family. My other guess is that he's jealous of you. Look, when it comes to visiting with families, it's natural that family members will show interest in you and reminisce. A pwBPD often feels excluded when attention is on you and not on him. The mere act of you engaging in a conversation with a family member feels like "abandonment," because (i) your attention is temporarily diverted away from your husband, (ii) you are "stealing" attention and (iii) you're having a good time, "excluding" him. Any of these issues could be triggering. Does your husband seem to lose interest in chit-chat, drift away from conversations and then accuse you of "excluding" him? Does he think your family is "mean" to him and "ignores" him at family gatherings? It might be because he's misreading your happy family updates as irritating and demeaning to him.
Another phenomenon happens in family gatherings with the pwBPD in my life. She's extremely insecure and generally unhappy, so she's totally disarmed by typical, well-meaning questions such as, "How are you?" and "How's work/school going?" You see, a question like "How are you?" ends up reminding her of her unhappiness, in addition to pressuring her to act "fake nice." A question like "How's work?" feels like prying and harsh judgment, because she's insecure about being unemployed or underemployed. I can practically see her fall apart inside as she deals with her negative emotions and can't come up with natural responses to simple questions like these. And you know what? She's also socially awkward, as she hasn't seemed to learn to ask anyone else any questions. She shows ZERO curiosity about other people's lives, and conversations with her just seem to peter out quickly. She'll answer questions with one-word answers but neglects to pass the conversation ball back.
I'd say, don't rule out that your husband could be straight-up jealous of you. Maybe when you're with family, you seem to glow and exude happiness. You seem to light up the room with conversation. Your husband might think, why is she not like that with me? And his negative thinking will take over. When visiting with family, you might try to make sure to talk with everyone and catch up, especially if you haven't seen them for a time. When you do that, your husband could take it to mean that you're neglecting him. That puts him in a bad mood, and he might withdraw from the conversations, which only magnifies the feeling. Does that make any sense in your situation?
Just my two cents.
You really hit the nail on the head. While he doesn't say they're mean, he does complain about being ignored. People will say hi, ask him general questions about SS or about trips we have planned. Sometimes he's drawn more into the discussions, but sometimes that's it. He complains about that. He also complains how they talk about people or events he's not familiar with. He always tells me that his parents taught him you never talk about people or events that aren't familiar to other people in the conversation. (Ignoring the fact that he and his mother and his friends do that around me all the time, though I don't particularly mind it. I just listen and ask questions.) I fill him in on things, but he still complains about it. I think it's natural for people who've known each other for a long time. You just bring the new people in on the stories.
He probably is jealous of how happy I can seem around them. He's jealous that I seem to have good relationships with my nephew and nieces, but don't have much connection to SS15. Anyway, I've tried to draw him in to conversations, but it's like trying to push molasses up the hill.
But I think he's also jealous because of what I have. He's shared before that he's always wanted to be part of a loving family. It's not something he's ever had. I'm one of five kids, we're all very close to our parents and (to varying degrees) to each other. I grew up in a town with all four of my grandparents and we saw them multiple times a week. He's expressed feeling inadequate because I know all these old family stories and I know all my grandparents' birthdays. Well, I know because I grew up celebrating them. He loves crime shows and mysteries like I do, but he's also always been drawn to shows about big, loving families. I think he really, really wants to be a part of mine. But he doesn't know how.