Is it normal for me to be obsessed with my boyfriends sister?

This is in a negative way and I'm a girl.

She's younger than me by 2 years. I'm 22, she's 20.

I just dont like her, but she calls me often and she asks to hang out at least twice every month, but I just hate her.

She is anorexic skinny and I lived with her an entire month one time no lie and I saw she skipped lunch every day ate dinner around 9 and went to sleep. She must have had less than 800 calories every day. She's 89 pounds and 5'6. She can't even fit in 00s sometimes she has to always wear belts. Her spine shows, etc and this annoys me because I tell her to eat and she ignores me.

Anyway, her hair is always greasy because she only showers once a month because she says her hair is healthier that way, but her hair IS MASSIVELY GREASY WITH DANDRUFF..

anyway, she likes dance music with ccrazy shit beats and hates music with lyrics - this annoys me to death.

she moved to florida at age 18 to be with her boyfriend of 3 months. she never went to college, never held a job.

she always buys things all the time, they have an apartment filled with thousands of dvds that she has bought within the past 3 years. they arent wealthy and are barely making it.

is this normal to hate her even though shes nice to me?