Hire an experience criminal defense attorney immediately. If a warrant has not yet issued, and if you are in a jurisdiction that has pre-warrant probable cause hearings, it may be possible to prevent a warrant from being issued. If a warrant has already been issued, then it may be possible to intervene with the local prosecutor to stop this case before formal charges are brought.
I agree with Mr. Hasson. Charges may never be filed and it could simply be an idle threat by your sister. The police and prosecutors may also not believe her enough to actually file charges. I would consult with a local attorney though since it is better to protect yourself in case something does end up happening.