Generalized sufficiently, all of these words mean "relative". Consider the weakest relationship between words and thoughts: the words that you habitually use g

Generalized sufficiently, all of these words mean "relative". Consider the weakest relationship between words and thoughts: the words that you habitually use guide/reflect how you think of the world. If the social relationship of "elder brother's husband" is different for you than "younger wife's sister" in some generalizable way, then words like these are meaningful to you. If you don't have any preconceived notion that you should be more polite or defer to one of those more than to the other, maybe they are just labels. Cultures and families are different. When they adopt words, they usually have meanings.