How do I get my sister (29F) out of her victimhood mindset?

I'm not talking about political victimhood like discrimination, I'm talking about being jealous or thinking that people treat the others better than her.

She's 29 and she still throws tantrums when my other sister and I get something from our parents and she doesn't. It's kind of insane since she still acts like a child as a full-grown adult.

Since her teens, she always thought that my parents treat her worse than they treat us. There might be a little truth to that but when she speaks she really acts like she's Cinderella, she threw a tantrum when my dad helped me buy a new computer even though she completely bought her phone from his money, she threw a tantrum when she didn't get in med school and I (and my other sister) did and she blamed that on dad's pressure and being unsupportive, she throws lots of tantrums about different puny things everyday.

3 days ago she was angry and threw a MASSIVE fit because we only left her one cookie for breakfast even though she's obese and she isn't even supposed to eat cookies.

How the hell do I fix her?

tl;dr: my sister is very immature and thinks she's always the victim and I want to get her out of that weird mindset.