Boyfriends little sister....help!!?

shes 14 and im 18 along with my boyrfriend whos also 18. ive always been nice to her and we used to get along great however now my boyfriend and i have been together a year and she randomly started to hate me and calls me names behind my back and throws me dirty looks all the time. she has issues because shes really spoiled and wants everything her way no matter what. one time when she made her dad help her with her homework she lost it and yelled "i hope your blood pressure goes up so high that you die." she has no compassion for other people. yesterday my dad died from brain cancer and my boyfriend and his parents are going to the funeral and wake. they told the girl that she was coming but she threw a fit and said "i dont want to go its ******* retarded". im really hurt because i havent done anything to her and shes attacking my family and my dad. i dont know how to handle the situation. her parents dont discipline her and usually let her get her way because theyre scared of her i think. what should i do?