Having a bit education in psychology, I would never say this is true. If your sister is really hiding something from you usually there are more common reasons like
a) It is pretty bad, and she is ashamed to tell you
b) She thinks you will not understand and that you would be judgemental instead
c) -related to b) - Does not expect to get support from you
Try a different approach. Just be a friend to her. Listen. Don't judge. Don't blame. No matter how bad it is try to be supportive. After all, she is your sister. Try to be someone she can lookup for help regardless of the situation. I'm sure she will appreciate it. gl
ehh