Trying to avoid a probate IHT mess...

xylophone

with the proviso that if B does not survive A by 28 days then the estate residue is split equally between the children (which is then itself a problem because there's no detail on how it would be administered for the 2 minor children).

No problem in so far as the two minors cannot give good receipt/hold real estate in their own names and therefore the default position is a bare trust until they reach their majority.

Keep_pedalling

Oh, the joy of DIY wills. I think you need to take legal advice to make sure what the testator wanted happens. As it stands B inherits, so what happens to the bequest if he dies, runs into financial difficulty or has to go into care? 

xylophone

As it stands B inherits, so what happens to the bequest if he dies, runs into financial difficulty or has to go into care? 

B has made a will (  OP refers to being B's exor in the fullness of time) so that the property would pass according to his will as part of the residue. 

With regard to B having financial difficulties/going into care, it seems that he has a  substantial estate (not including the value of the property under discussion).

B is of a pretty advanced age, has assets that would already put their estate above the IHT threshold even before they took ownership of this additional property; 

xylophone

 I think you need to take legal advice to make sure what the testator wanted happens.

As things stand, the  (perfectly valid) will is clear as to the beneficiary - what the testator said to the beneficiary as to what he hoped would happen is neither here nor there?

That said, I would have thought that the children or the OP acting on their behalf could demand "reasonable provision" from their parent's will?