I can see it going either way, with no way of us out in internet land knowing which is correct.
I can see him being the son that isn’t favoured. And has only recently realised it, perhaps with the help of an outside perspective. It happens a lot. I can also see his wife (with her own perspective) framing the relationship with his mother as ‘second-hand’ since it doesn’t present in the way her experiences tell her is loving. Who knows what the truth is?
I would say though - if you feel like someone doesn’t have much time for you, and never makes an effort for you, I would match that level. While I would still attend functions, be happy, bring a potato salad, etc, I wouldn’t drop everything to help them with something if they’ll never help you. Basically, most healthy relationships amongst adults are reciprocal. If you honestly believe your mother is all take and no give, then I would stop expecting any give so you don’t get hurt by unmet expectations, and focus more on people who bring good to your life. That doesn’t mean cutting her off, just a little bit of emotional detachment, so you aren’t chasing something from someone who can’t give it to you.
And your wife needs to back way off. It’s your relationship with your mother, so it’s up to you how you want to manage it (absent abuse and toxicity, of course). It sounds like she’s not getting the relationship she wanted and expected to get from her MIL. That’s sad, but she needs to accept it, because it is what it is and her MIL doesn’t owe her a certain type of relationship. While I think you may only need a teeny bit of emotional detachment, it sounds like your wife needs a dump truck full! She can’t force a type of relationship, and if she can’t accept the more hands-off relationship with her MIL, then she needs to back off and have a minimal (though polite and civil) one. And look elsewhere for the closeness she seeks