A parent (of 5) with dementia (ie not going to be changing their will) has a will to divide a home property between 2 and the residue between the other 3.
So, one property left to two children and estate residue left to other 3?
One of the 2 (who will receive a joint share of the property) is now terminally ill and is leaving all they have to nieces and nephews (no kids or spouse).
They can only leave what they actually own, they cannot leave the share of the property that is willed to them, because it is not theirs if the parent is alive when they pass.
So three questions;
a. if the child dies first, where does the joint share go?
No children and no spouse - I am not a legal expert, but to my mind this goes back to the estate to be divided between the other children.
EDIT: If child dies before person who willed property to them, then child cannot will what they have not received.
b. if the child dies second can the other joint owner be forced to sell (to get funds for the care of the ill child)
If the parent dies and leaves half of property to the second child, who in turn wills it to the nephews and nieces - then to the best of my knowledge the joint owner has to either have their permission or have the funds to buy them out. Each owner cannot be forced to sell if they do not want to.
c. is it possible in any way for the property to be sold if desired to cover the care costs of the parent? There is no power of attorney in place to activate and the children are unlikely to agree to a collective effort to ask a court for power to sell.
The only thing I am thinking of here is the fair deal scheme, if a person went into a state run home then they could have come to an arrangement with the state that on their death and on the sale of their home, their cost from the home could be deducted from the sale.