I might not be popular with this, but can I throw in another viewpoint?
My split wasn't amicable although on the surface it was - as in, no arguing. But my XH is a lying cheating piece of shit.
From one side, you could say that there was a bad divorce and he hasn't bothered* with seeing his kids regularly for TWO YEARS. That's a lot of day to day childcare that's fallen to her.
*i say hasn't bothered - I'm not trying to insult your brother, I'm just saying how this could look - especially how she may have built it up in her own mind. The way you say it sounds like she offered little, and he took little.
And even now he has weekends, he's not changing his work pattern to see them. She may be awkward, or she may genuinely believe that midweeks are unsettling to routine. I don't think they are - but have reasonable friends who do!
With my ex, actually he has our child weekdays regularly because of my work. We have a non fixed pattern because my work location is variable. Weekends were always "on request" and no request ever turned down.
He never requested to see her at the weekend in EIGHT MONTHS.
Then along came a girlfriend. And (IMO) he wanted to look like Perfect Dad. I don't think I'm just bitter. He split up with her and stopped asking for daughter, got back together and instantly wanted her again.
So look - from her point of view, why SHOULD she give up Xmas, special times with her boys for a man who has been largely absent for 2 years, and (maybe from her point of view) just wants to play happy families now he has a lovely girlfriend?
Of course - she SHOULD (and the law will back him) because it's right for the CHILDREN.
But don't underestimate how hard I gritted my teeth when I offered Xmas Day because XH was going to have big family thing including new girlfriend and her child.
Imagine the XW posting "why do I have to sit on my own on Xmas Day now he decides that after barely seeing them for two years, he wants them?"
She'd be told to pull herself together - but she'd get sympathy too.
Even the kids party thing... Birthday parties are lovely for kids and part of their social life. I had a boyfriend who was gutted to see his son missing out because XW refused to take their son on her weekends. So I can see that could come from a reasonable place.
End of the day, yes for the kids sake he should have much more and more frequent access.
I just think, be careful without the facts not to vilify the ex.
And definitely think about changing the weekend night working.