@ChiCricket my parents divorced when I was really young, and my dad married my stepmother when I was three and her son was four. Then they had three children together. So I have, technically although we never differentiate, a stepbrother and three half-siblings. I spent most of my time with my mother in a different state.
For me, there is a distance because I spent most of my time with my mother, so there is a lack of shared experiences. But I don’t feel any different from them because we aren’t all full siblings. And we are all closer as adults. My middle brother and I favor each other heavily, as do our children. I will note that my sister does draw a distinction between the full three and my older brother and me.
So, I wouldn’t tell the brother and I would leave it be. It’s really hard to be the odd one out. Maybe on some level he suspects. People get weird with that info. Like, what if someone refuses to give your niece a family heirloom because it’s from the dad’s family. I think there’s too much risk of stirring up unpleasantness for him and others. If he is curious, he certainly knows he can ask the daughter.
That being said, wow! Sorry you’re going through all of this.