Why do I feel like I'm being avoided by her?

I have a sister who is older then me. I call her, email her, leave her voicemails at home and on her cell. And she never replies. I know her relationship with our mother and father is not the best and she doesnt keep in touch wih them either. We live about 20 minutes from eachother and we see eachother maybe 2-3 imes a year. Ive asked her before if she was mad at me and she said no, she was just busy. She dnt work, dnt go to school....... What shuld I do? Should I just stop contacting her? Please help me.