Land Registry Info/advice?

Hi There , 
My father passed away may 2018, and I have filled all of the relevant forms etc to have his name removed from the land register, leaving just my mum as the sole owner.
Whilst we were going down the ugly route of death, I helped them both out by paying off their debts to the sum of £30k and in-between may 2018 & now I have helped mum out with some further issues to which the whole debt owed to me is about £40k. I did all of this obviously as a good son!! but more importantly to give her the financial stability to be able to pay off the rest of the mortgage (£20k remaining) on their home and not struggle with the burden of the other debts on the proviso that I would be placed onto the land registry along with my mum as Joints Owners in common (think its that one or its the other) and I would have  50% on paper.

So this should be pretty straight forward to do?, however there is a larger issue that will arise from my mum (stepmum!) once she passes, she has a daughter (not my fathers) whom is well let's put this politely , has 4 children from 4 different fathers, has been in and out of her majesty pleasure as a drug dealer, and to put it mildly is a nasty piece of work.
Once she hears that mummy has gone (they haven't spoken for at least 15yrs!) she will be down the solicitors trying to get a share of her estate. 
Now to get around this we have had a will drawn up for my mum stipulating in the will that the daughter is entitled to absolutely nothing and has also hand written a letter to be given to her when the time is right explaining why!!

I have another older brother, who is aware of all of this and currently the will is set that I will receive a 50% share of her estate and then the remaining 50% will be shared equally between my bruv and I.
So my question to you all is: 
Am I going about this the right way re: land registry 50/50 split?
Is the will and a signed witnessed letter enough to make sure that she doesn't get a thing in a court of law?
All very disjointed I know but any advice would be great please


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