Yes I do think it was ok Diane.
'Healthy' people didn't actually know they were ok and quite often they weren't which would increase the risk to others. So no obviously they shouldn't be going to holiday homes, using the local services and potentially spreading it around. Self isolating in a house isn't a holiday and the rules were there in place to stop people from using the sudden free time /properties for that.
Unless there's evidence to the contrary this isn't what DC did. He didn't go out. He wasn't on holiday. He went to a home that was on the private land of his parents farm, he isolated there and didn't come into contact with others. Food was dropped off for him on the doorstep. His reason for going was so that his child could be looked after if he / his wife or both got too sick to care for her.
Yes maybe the sister could have come down and collected the child, but we don't know her own family commitments and whether this was feasible to do this. And the time it would take to travel down and collect said child may have been very problematic if their health had taken a rapid and significant turn for the worst. Clearly it would be better for the child to be with family if this happened.
I don't have a problem with anyone moving in these specific circumstances and under the conditions DC said it was in.
What do you think he should have done if they had stayed and had to seek urgent hospital admission or simply couldn't look after the child?