This question is about the complexity of family relationships and titles. In traditional Chinese culture, family relationships and titles are very important, they are not only a matter of titles, but also an important bond to maintain family relationships.
First, let's analyze a few of the main character relationships in this question. The brother-in-law refers to the wife's younger brother, the sister's husband refers to the brother-in-law, and the sister's husband's parents refer to the brother-in-law's parents. Therefore, how the brother-in-law calls the parents of the sister-in-law's husband refers to how the brother-in-law calls the parents-in-law of the brother-in-law.
In traditional Chinese culture, the brother-in-law should be called "uncle and aunt" for the parents of the elder sister's husband, and some are also called uncles and aunts.
In daily life, the brother-in-law can directly call them "uncle" and "aunt", because the parents-in-law are also the elders of the brother-in-law.
In conclusion, family relationships and titles are a complex and interesting topic, and different regions and cultural backgrounds may have different ways of calling and cultural Xi. However, family relationships and titles are important ties to family harmony and social relations, regardless of the location.