It just goes to show that people want time off over Xmas for all sorts of reasons. To them, their reason matters just as much. You could say that Xmas is about family in general and togetherness, not just DC's, and her DM certainly is that.
Ultimately whatever the reason, nobody should miss out more, and it should be shared. Unfortunately with Xmas, and time off or rotas worked for any reason, it will always tip in the favour of those who speak loudest, so you worked the late on Xmas eve lots most likely as you didn't request or complain as vocally to the powers that be who set the rota.
But one person's experience of disgruntlement is all relative, so given that I've worked the late on Xmas day last year, and am doing it again this year, perhaps you could count yourself lucky that none of you have had to complain about working Xmas day? - Xmas eve being just another ordinary day, and no extra pay either for some, so it's neither here nor there. Lol, had to get that in as arguing about eve when working the dad seems small potatoes.
The real overall upshot, is, it is what it is and in life some people can't get over things, even small things, these are just 2 examples, it happens. But that's about them, and shows who they are, not about you. Your issue is in taking it personally, whereas most people just shrug their shoulders and get on with life. You are most likely not a common denominator, this is just a case of 2 out of hundreds you've known in life. Also, MIL situation is a notorious one.