Well, Suzy, I am still struggling to see anything other than a jobsworth approach in your replies. It depends entirely on the assessment of the degree of sensibility and maturity of the young man concerned and his relationship with his sister. What I am saying is that the idea that he magically changes in his attitudes and capabilities at midnight on the night before his 18th Birthday to being old enough to look after his sister, die for his country etc seems a nonsense. Many young people display great responsibility in all sorts of situations, some of which they ought not to have been placed in, but they have handled it as well as any so called adult. the only way to learn responsibility is to practice it, and thank heaven there are still people with the courage to let young people have the opportunity. We have seen increasing applications of legislation to try and micromanage every likely situation and yes, you may sneer at other people's views, but the whole ethos of CRB now being introduced is that everybody is suspect, and it is destroying trust.
So I say to the parent rasing the question, if you feel your sone has the maturity to handle this for a couple of days and the right relationship with his sister, then assess the risks of it going wrong and if you think this is reasonable, then ignore the "You do this and we'll prosecute you" brigade and do it. I am sure your children will be pleased for you and behave exactly as you would wish them to, so as not to spoil your short trip away, an appreciate being trusted.