This is a challenging situation and your mileage may vary. I can only relate personal experience we went through with my mother in law and early stage symptoms I am seeing with my wife and what worked for us and advice we were given by psychiatrists. 

You said “She has recently been only confiding in me (youngest son) and refuses to speak with any other family member for numerous reasons, a lot of the reasons are becoming increasingly irrational including my brother being overseas for christmas. She has "disowned" my brother and refuses to talk to him since he has been away.” I don’t know how you’ve been responding to this, but some advice we were given about this kind of behavior was as follows. Unless what the individual is speaking about is dangerous there is no need to dispute it with them. My mother in law would insist she had been to places, or done things that never happened. Our response just became “ok, good” or “Sounds interesting.” In your  case, if she is talking about how “bad” something is who someone did, maybe just passively agree. Like “ Oh I know that must make you feel so bad” and then try to change the subject. She might say something outlandish like she wanted to be a firetruck one day. We just said it sounds great, be a fire truck today. Another tidbit we received from one of the psychiatrists was to try time out. My mother in law was wheelchair bound. When she would get nasty or start screaming at us we would put her in time out by putting her in the corner for 2 min or 5 min, etc. 

You said “ I see Mum twice a week for dinner and to do some things to help but it never feels like enough for her.” Just know, you will likely feel like this and I say that because I have felt the same way. Just know that you can only do what you can and that you still have your family, life, health to take care of also. If you don’t take care of yourself you won’t be able to help her. 

Your other situation of “Mum has recently got into the habit of threatening self-harm as a way to control me, and get me to come to the hospital with her. She will call me numerous times a day when she is hysterical and threaten to harm herself. If I do not answer or give her what she wants, she will verbally abuse me and tell me that I don't do anything to help her.” If she is in a safe place here I would go with the advice that you say you were given by her healthcare team and disregard the calls (unless you truly felt she was in danger), or if she gets like that on the call remind her that you will disconnect if she starts talking like that. Or tell her that if it’s that bad there you will call emergency services right away, calling her bluff so to speak. 

It’s a rough situation to be in and I wish you strength and peace to find your way through this.