What does Xinhua man mean by calling himself a woman's sister-in-law?

It is normal. Generally speaking, being away from home means that you know each other or your brother is older than yourself, so if you know each other, you can call your wife and sister-in-law, which means sister-in-law.

In fact, it is a very polite, normal and common greeting mode, or a title.

But this is the special name of this place. If you don't call your wife and sister-in-law, you call someone else's sister-in-law. This is the case in many places.