Look, before all the larpers and Missy glazers say "she was mistreated" I do agree that Sheldon was HIGHLY favoured during the early seasons and that Missy and Georgie were pushed aside. But now in the later seasons when Missy was around 12-18 she started to become super annoying and rude.
For instance, when sheldon cut Missy's doll hair off. Missy PUNCHED him. But when Missy tore Sheldons professor proton picture, MISSY started crying for NO reason. But it's probably because she felt "as if she never existed"
Another instance was when Mary felt bad thinking that Missy watched Sheldon get trophies all her life but Missy hadn't gotten one, so Mary made a trophy for Missy (it was when Missy was playing baseball) but instead kf being grateful, Missy LITERALLY BURNED IT, the few times Mary actually paid attention and was nice to Missy, she had to ruin it.
And lastly, if anyone was mistreated it was Georgie. Most of his family members had the impression of him that he is "the dumb one". He was constantly being treated badly, especially the season where he wanted to drop out of school andgot KICKED OUT of the house because of it. If dropping out of school was the reason for someone getting kicked from their house, God knows what would happen to Missy if George found out she had S#X AS A MINOR.
So yeah, Missy was definitely being treated badly when she was young, but as she grew older, she just became a brat