My dad died before I could pay him back $20,000 that he lent me.
My dad had a Will that left 50/50 to my sister and myself. $1,000,000 each.
My sister changed my dad's Will when he had dementia and he had no idea what he was doing.
My sister ended up with all the money being $2,000,000 in total.
Do I have a moral obligation to give me sister half of the $20,000 that my dad lent to me that I never repaid to my dad?
Tough decision. If the father were still alive, even with dementia and even if the sister had altered the Will, I think you would owe him the $20,000 due to the (I presume) promise you made to pay him back. The promise was made to him, after all, and was probably not qualified in terms of mental fitness ("I will pay you back so long as your are mentally competent"). With his passing (and I am sorry for your loss) I suggest that matters change insofar as your sister manipulatively (wrongly) altered the Will. Ideally, the sister might have a moment of conscience and, realizing the wrong she committed, she would voluntarily half the bequest. Perhaps (from a strategic point of view) offering her the $20K might even shock her into some repentance, e.g she might be incredulous (in a good way) that while you have been wronged, you were still trying to make amends. Failing that, there might be legal recourse of declaring the Will null and void, given that your father was not competent to make the change he did...