Upvote:5
The official site seems pretty clear:
When travelling to the UK with an adult (someone over the age of 18) you’ll need to identify them in your visa application.
Their name will appear on your visa, and you’ll be refused entry to the UK if you arrive in the UK without them.
A child visa can definitely be used to stay in the UK after one's 18th birthday. I guess it could also be used to re-enter the UK, although I am not sure, but it doesn't matter here as your sister-in-law does not have a visa to travel alone.