My brother's wife is called sister-in-law or sister-in-law. Sister in law is a more written name, which is obvious, that is, the daughter-in-law of the younger brother. Younger siblings are more traditional names, which sound more friendly. But today's young people don't pay so much attention, and their brother's wife can call him by his first name.
In some areas, it generally refers to the wife of a younger brother or the wife of a brother who has a very good relationship with himself (of course, it is younger than himself, otherwise it should be called sister-in-law), that is, sister-in-law. Sister in law and sister-in-law basically mean the same thing, but in some contexts, "sister-in-law" can mean "brother and sister". For example, "I have a father and mother who are over 60 years old and a younger sister-in-law". This is the difference between sister-in-law and sister-in-law.
Sometimes it can also be used as the name of a colleague's wife (of course, the premise is that the colleague is younger than himself), which can not only respect people, but also avoid the embarrassment of the relationship between friends and colleagues and her. It can generally refer to friends, girlfriends or wives younger than themselves. Generally, there are many people living in the north.