You are supposed to pay for looking after your parents when they are old, according to French law. If your father doesn’t want to come back to the UK how will you force him to?
If he hasn’t got dementia his wishes apply, not yours or the stepchildren’s. Of course if you don’t live in the same country it can be more complicated. Do the stepchildren also have families to look after? Are you expecting them to take care of him? Did he work in France?