Is one of the two older sisters the one for whom the original UK house was bought? And now she's refusing to help out? Sweet! Sounds like a bit of a "King Lear" situation going on.
It sounds like your father has enough assets to cover everything. It's just "a cash flow problem". You could consider lending him the money. BUT: engage a solicitor, make him pay for the costs of that, make sure the loan is secured on his assets, make sure this is reflected in his will (which needs to be drawn up if he doesn't have one), make sure the agreement prevents him from reducing his assets below a certain level, make sure he understands that the assets would not be available to cover (for instance) Fair Deal, equity release schemes, or anything else that he might want to avail of in future. There's a reason banks don't want to go beyond a certain age -- people's needs change.
Basically, sew it up as tight as any other sensible lender would, or else let him take his chances with the bank. Oh, and after all that, you're still taking a big risk, so consider something to cover the intangible cost of that too, perhaps an additional consideration in your father's will. If it all sounds massively messy and complicated ... it is.