One cannot force anyone to accept the truth rather the important point is that you put the proof upon her. If she refuses to hear anymore out of stubbornness, then you’ve done your job and have no blame. Those who are stubborn should be left if one sees that there is no benefit in utilising anymore time with them.
Furthermore it is not recommended to take an approach where they feel that you are trying to force them. Usually the ‘overly pushy’ type of approach makes people further away from religion.
The best approach to take would be one combined of showing good ethics and respect. Know that helping one to the path of guidance can sometimes take years. Therefore, always have hope that she may take interest in future and come to you for knowledge.
There will always be a time in someone’s life where they think about God; this may occur due to a particular incident such as a death of a loved one or near death experience. It is however upon the person to go deeper and search.
Also the below verse is something for us all to take into consideration when trying to propagate our faith.
“Call to the way of your Lord with wisdom and goodly exhortation, and have disputations with them in the best manner; surely your Lord best knows those who go astray from His path, and He knows best those who follow the right way.”
May Allah grant you success and guide your sister to Shia Islam.