I had this with my gay DS a few years ago, it was when JKR first stepped in to the debate. I think the reason his generation dislike her so much (he is late 20s now) is that she was an absolute heroine in their eyes. We had Harry Potter everything in our house as my son was a reluctant reader (ASD, Dyslexia) and the HP books revolutionised his view of reading.
He has spent 10 years in academia, Oxbridge, Doctorate, etc and he has known nothing but hatred for her. I think that the whole LGBT+++ gives him a feeling of belonging and a friendship group, and he feels betrayed by her. I still find his views on this whole subject quite incredible as he is a scientist.
Anyway, we had a blazing row and didn’t speak for six months, which broke my heart. I was particularly upset as he referred to me as an old woman who didn’t understand the modern world and I should educate myself or keep quiet. This from a young man who has two sisters and was brought up to value and respect women. My DH brokered a low key reunion.
We are back to normal now, I ignore his social media feed which is pretty much nothing but trans support, with the occasional general rainbow post. I of course don't post anything pro GC, because I have too much at stake to risk it, yes my job, a couple of my younger friends possibly, but most of all of course my son. He must know how I feel, but it is just a no go area, and I keep quiet to keep my son.
His sisters are younger and less concerned, one has a low key, be kind attitude, which is how she is about everything, but no obvious support for TR. The other eye rolls, but keeps quiet at college as she knows how damaging it could be. I have no idea if they all talk about it when together.
How you proceed now depends on what relationship you want with your nieces. I do think it’s a shame your brother didn’t offer any support, but I get it, and it may well not have helped. My DH made it clear at the time of the argument that he holds the same views as me, but the vitriol was all aimed at me. It is as often said on here a men’s rights movement, with us Old Women seen to be hanging on to outdated rights.