Thanks for choosing health care magic for posting your query.
The kind of behavior you are describing appears to be part of a personality trait (Feature) and not mostly a illness. It can be some personality disorder
but then she needs to be examined by a psychiatrist
. We don't know the exact age of your sister. that would have helped us in providing you a better answer. If it is adolescence, then children at this age do exhibit this kind of behaviour but it is actually very transient and will reduce as she grows old and becomes more mature.
Hope I am able to solve your query.
In case you want a direct and detailed discussion about your problem, you can contact me on the following link:
Wish you good health!