What age is to old?

In Australia the age of consent is 16 so he can not be charged with anything. That being said I don’t believe a mature and well balanced 27 year old would normally be interested in someone that young. I also think that your sister-in-law is too young to realize that this relationship is strange and might even like the fact that he is older. The important thing is that she remains aware that he is older and more able to manipulate her into doing things she might not feel comfortable with. She should be aware that he might use his age as a way to control her or use her therefore she should be very alert if she feels that something isn’t right. She should also try not lose contact with friends her age. It’s important to keep a good circle of friends not matter what because they’ll always be there for her. If you are friends with her then you could just voice your worries and talk about it. Remind her you’ll always be there if she needs a friend.

That is UN-SAFE for her at the age she is, maybe if the guy was 20 years old MAX. she and others would have nothing to worry about.