My fiancee's, brother's wife is a complete wack job...?

She told me and her husband (the brother) she liked my fiancee, she accused me of trying to sabotage her wedding (they didn't have). She told me she would always be jealous of me because I have my fiancee and his daughters. She tries to outdo me in everything, tries to be smarter than me (even though I am the one in college). I try to be nice and ignore her stupidity (be the bigger person) and she is nice (sometimes) but most of the time she throws temper tantrums to get attention. She handed her son a lit blunt and smiled and thought it was cute :( she leaves her daughter in a binet changing table to sleep in during the night, propped up with a bottle and thinks nothing is wrong with any of this. When I or someone tries to tell her she's wrong she gets mad and throws her tantrum. What is wrong with this girl?