Why do some people find it's hard to visit the grave of their loved ones?

My parents died in an accident 5 years ago. They were rushing to hospital where my sister was admitted because she passed out in her dorm. It was one of those moments in your life that couldn't be forgotten. My sister had yet to wake up when we buried our parents, and when she finally did she was just sat there motionless, and didn't say a thing. Until today she has never visited our parents grave. Instead, she signed up for a job that requires her to travel the world constantly. Though she keeps on checking on me and come home for christmas, even when we go for christmas service she doesn't even spare a glance at the cemetery (we have to pass it if we go to church). I couldn't understand her attitude, and honestly, I'm pretty offended by her behaviour, it's as if she doesn't give a damn about our parents. Do u think so?