There's a gap in this train for me, but isn't this about the decline of extended-family connections? I think liberalism could play a big role in there, in that it is inherently opposed to tribalism and nepotism, but
@Kafir-kun also provided a cultural-evolution-based explanation that seems plausible (which, of course, doesn't mean it's correct).
Throughout our history ("our"=humans here), we've developed rituals--food-processing, tool-making, divination, etc.--that no one engaging in them could explain the benefits of but that were actually extremely important to our survival. A liberal in most times and places could get killed--either by offending the tribe or my skipping steps in those rituals that didn't *seem* essential but that we now know are. But that last clause is important. We have reached a point in our cultural evolution where we can make liberal arguments for stuff described
here (and more in the book referenced). It might be that there are good liberal arguments for deepening extended-family connections.