Morning OJ - I don't post much but am on the prayer thread and we are all thinking of you all the time.
DH does a lot of terminal care in his role as a rural GP, and says that whet you are going through with family is unfortunately incredibly common - esp mothers trying to interfere and change the dynamic of the relationship when their child (adult!) is ill.
Steve's family have no right to any info from the hospice etc as YOU are his next of kin - they will simply ignore BIL's letter, or reply saying that he has no right to info.
You are being amazing - though I am sure you think you are just doing what anyone would do. I bet you are incredibly proud of Steve as well, insisting on your presence, teaching his Mum a lesson even when he is so ill - he obviously loves you so very, very much if he can reach out through all the confusion in his head to stand with you through all the crap his family are giving you.
My mum abandoned our family when I was 12 - I wouldn't want her anywhere near me if I was even as sick as Steve is. He's an amazing man and I am proud to have "met" him through MN, as well as you.
The love and support of all of Mumsnet is behind you, I can tell!
lots of love xxxxxx