OP, if it applies. I had a close relative who liked to drive wedges. I was victim to it and saw her do it to others all my life. Grossly favoring one sibling over another within families, taking one spouse's side over the other's and making a bunch of noise about it when the couple wasn't even having problems.
Everywhere she went, she started this crap, through many years. She was good at it too, working behind the scenes and turning people against each other. It's hard to understand when you don't think that way yourself. I guess it would be some kind of mental problem or personality disorder, or just some weird need for power, I don't know.
Since then, I've seen that, unfortunately, it's not that uncommon for a troubled person to do this. So if these brothers' parent or someone else is behind your daughter not being invited to two different weddings, this is possible. And if so, your daughter not being your husband's biological child would only be something they found that they could use for this weird purpose. If it wasn't that, they'd have found something else. That's why I said it's not surprising the younger child is the one being favored and (possibly) used to stir up mess within your family. She's the easiest way in, to be able to drive a wedge within your family, in other words. The wedding exclusion being repeated a second time with a different brother is what made me think there may be one of these people behind it all.
If you don't know about this rather common dynamic, you could look up things like "triangulation," "golden child/scapegoat" and so on, and see if you recognize a pattern. I don't know of course but your situation just has a familiar ring to it, to me.
Anyway, this person decided to start another round of sneaky wedge-driving, using my grown daughter, who is mildly disabled. She became close with her and started trying to turn her against me and oh, just stir up whatever she could for no reason at all, yet again.
Even though my child was grown and had become close to this relative, I asked her to cut ties. I explained why, the best I could and reminded grown child that she (grown child) and I are the closest by far, not outsiders who try to cause trouble between us, even if they are relatives, too. My child trusted me and did cut the tie, even though she was grown and I couldn't make her do it.
But if she had been a minor, I would have made that decision for her because I know this person a lot better than she does and I know the whole deal through many years. Whereas all my kid knew was that this person was "being extra nice" to her. She'd have been a sitting duck, in other words. I'd think of it as my job to keep her out of that kind of miserable game. If this is even on the right track... Anyway, best wishes to you and your family. :)