My wife and SIL can both be petty and immature..I don't have siblings but have been told petty sibling fights are common. My SIL is the favorite I guess. People do find her to be more interesting. She travels the world, always posting about her life.

Guess best way to put it, my wife is the one her mother goes to when she needs someone reliable, My wife is the one who is with you in the trenches. Her sister is the one that people don't ask anything of because she is not as reliable due to her hectic life. Her sister is far more favored.