29M, 27F, 3-year relationship: how do I get my family to stop being shitty to my partner?

My family has been pretty shitty towards my partner for a good chunk of our relationship, but never in super outward ways, it’s more like a perpetual cold shoulder. It’s always the absence of kindness and genuine interest over the presence of overtly negative behavior, which weirdly enough I think I would prefer. I never know what to do in terms of how to stop it: for context, I think my family infantilizes me (I’m the youngest, so I think that makes it worse despite the fact that I’ll be thirty in less than a month.) I feel like if I try to call them on it directly, I’m going to look crazy and they’ll think “oh she’s the reason this wedge is being driven between us and our baby boy” or some bullshit like that, but I can’t just stand for it anymore.

Mind you, it should NOT have taken me three years to get to this point; I’m culpable for a lot of the harm that’s been done by not addressing it. I feel like I just flat out haven’t noticed a lot of the things they’ve done, but that’s not an excuse, I acknowledge that. I’ve been a fairly passive person for a lot of my life but it’s important to me that I change that habit.

It definitely needs to be a conversation but I don’t have the first clue what that looks like.