Me and my cousin used to be so close, now she is a total b**ch?

I have a cousin who was like a best friend to me. We did everything together and we told each other EVERYTHING. But now shes acting all snooty and acts like her s**t dont stink. I invited her to go with me for some activity in the park. But she says (in a snooty voice) "no i probably have something to do that day". The last time we had a good time together was the day before my birthday (about 3 months ago) now its like she is different person . the preacher at the church died and i told her about the funeral, she said she didnt want to go because he was busy. (yeah rite) and yesterday when i shown her the card from the funeral and said (in the snooty/b-**chy way) ok, just put it there. oooohhhh. that pissed me off. I've tried my best. I invite her to go everywhere with me, but she always make the same excuse "i gotta clean the house" (but she never does it) She's like a different person and she is only 4 years younger than me (i'm 20 she is 16) I told my dad about what was going on. And he just says "stay away from her, if she wants to act like that, then you dont need to be around her" but how can i just stop caring about someone i've known forever. HELP