Just to point out something about a woman's nieces/nephews over a mans. Could it be that the woman is more proactive in seeing her sibling and their children than a man is? It's not the womans job to facilitate the man seeing his sibling and their children and IME, men rarely bother (that's not to say they never do but my family and friends, it's ALWAYS the woman who does the family stuff)
You could have a point @MyPatronusIsAUnicorn.
And I do think men often don't try as hard as women to do stuff and maintain relationships. It's like an argument from the other week where it was daughters vs sons; the consensus from many was that daughters are more likely to keep in touch more, and make more effort with parents and family, whereas sons are a bit more blah...
However, my friend DOES try maintain a relationship with her brother, but HE doesn't make the effort really, and often puts her off when she tries to arrange stuff, by saying he and the SIL don't have time, and whatever...
So yeah what you say is probably largely true, but at the same time, it's my friend's brother that is at fault, (and the SIL to some extent!) and not my friend. And it's terribly unfair on her daughter (his niece,) as she is often excluded from stuff.
It's something similar that is happening to the OP sadly.
If it was me, I would speak to her brother. But I think that ship has sailed.