She is worried that the therapist is going to same as everyone else- It’s him that is going through a difficult time and she needs to support him etc.
Then she needs to check out Transwidows right now: and perhaps through them she can find someone to talk to.
https://www.transwidowsvoices.org/
I have a friend going through something similar. Her married son in his 40s, with children, had MH problems during Covid, resigned from his work and hasn't really recovered since then. Not long before Christmas he told his wife that he was a woman trapped in a man's body and wanted to express his true self, and since then has worn skirts and make-up and insisted that the children (still in primary school) call him by his new name. His wife was devastated: she was one of those people who had somehow managed to escape awareness of the whole GI/ transgender thing and just didn't know what to make of it. My friend, who has been GC for years, helped her DIL get up to speed.
When it became clear that the children were very unhappy with the situation the wife asked him to leave. I think probably a solicitor was involved. Certainly is now, because the wife has started divorce proceedings. The DS came home to live with my friend (his mother) who is struggling badly . He's very demanding and self-righteous and according to her he lives in his own transgender world full of trans online friends. She is very upset at how little concern he has for his children.
My friend feels strongly that he's mentally ill and she hoped his new GP would understand that and help get him appropriate treatment. Her son tells her that the GP sees transition as part of the healing process and doesn't think his MH has anything to do with it. He has been started on hormones.
The background and details of every situation are different, but the basics — the entitlement, the self-absorption, the lack of respect for wives and children — seem strangely similar. If I was in your shoes, OP, I'd support SIL and, sadly, stand well back from your DB. I couldn't be fair or impartial and I don't think you need to be. I don't think anything you do or say to him will make the slightest difference. As previous posters have said, he's been radicalised.
This programme from the BBC might give you an indication of how susceptible ordinary people can be and how difficult it is to deter them from doing things that are clearly, to the rest of us, inadvisable.
https://www.bbc.co.uk/iplayer/episode/m001krb2/a-very-british-cult