I agree with qiuzqueen.
Your sister in law is seventy years old. As a teacher she will be eligible for a teaching pension, and possibly she is already claiming it. She has also been eligible for a state pension for the last ten years. However much she enjoys her job, it really is time she hung up her chalk, or interactive whiteboard stylus. Young people need jobs, and a lot of unemployment is now expected through the Covid19 crisis. It is completely selfish in my opinion to hang on to a job when younger people need jobs. Obviously if there was a need for money, but as a retired teacher myself, I cannot see any way that this is the issue.