What can you tell about this girl who never visits her sister?

I have a close friend and she lives near San Francisco, CA. Her elder sister is married and lives an hour away from her. Now, my friend calls her sister and tries to visit her. Her sister appears to be busy and once in a while asks her to drop by. Sometimes, she bluntly asks her to be independent and respect her privacy. Now, my friend works in the finance sector. She is single, has lived in different locations near SFO and her sister has never, ever visited her home once or bothered to find out how she is doing. Its always my friend calling her elder sister up. Now, when I was visiting the family on occasion, I asked her why is it so, and the elder sister goes that she was never invited to come over. Now, I am a guy and I don't know how sister sister relationships work. But, is this normal?