16-year-old constantly steals her 19-year-old sister's possessions, parents buy her whatever she wants to stop her from stealing: 'My parents fund her Starbucks, skincare, clothes, and everything else'

Some parents treat their kids differently for easily decipherable reasons. If a mom had her first child at 18 and her second child at 34, she will almost certainly be a more mature parent the second time around. She also might be more lenient with her younger child than she was with her eldest, which could breed resentment. 

In families where one kid has a lot of medical issues, it's common for the healthy children and their problems to slip through the cracks because the parents are so hung up on keeping their sick kid alive. I'm not saying that these kinds of differences in parenting are good and healthy, but they are easy to understand, even if the realities of them are painful. 

There are, however, instances where parents favor one child for seemingly no reason. The child they favor is not more talented, hardworking, or smart than their sibling, and yet they are the one who receives all of the love and support that their sibling never seems to get. An unfavored kid could spend their whole childhood trying to figure out why they're treated differently and doing their best to win their parents' love and affection, but it's fruitless. If a parent decides to favor one child over another and sees nothing wrong with this imbalance, they are unlikely to change their behavior.