This is not a question that we can address here in any responsible manner. It is far too factually intensive and will require a proper analysis of any and all company agreements (operating agreement, subscription agreement, etc.) as well as all the facts and circumstances in context.
I suggest that you consult with a lawyer in private and discuss your options in more detail. You can start by calling around to several for a free phone consultation, get some insights then pick the best fit to work with.
DISCLAIMER: this is not intended to be specific legal advice and should not be relied upon as such. No attorney-client relationship is formed with the law firm of Natoli-Lapin, LLC on the basis of this posting.
There is no easy, cheap shortcut to getting an LLC dissolved if the members do not do it voluntarily. As a minority owner, you would have to bring a lawsuit to force a dissolution if your sister does not agree to dissolve, which would likely be very expensive. And even if you do file such a lawsuit, the judge might not dissolve the LLC if it is a going concern. If, as you imply, the restaurant would be forfeited back to the original owners if the LLC is dissolved, it is very unlikely that a judge would grant a dissolution unless your sister ultimately agrees.
There is no magic bullet to get you out of the situation you are in. You will need to hire a lawyer if you want to get this straightened out properly.
The other two attorneys that answered before me have given a good general appraisal of your situation. You have a mess on your hands unfortunately. You will need a good civil litigator who is also very well versed in business entity law to help guide you through this. Good luck.
This answer is based on general legal principles only and is not intended to provide specific legal advice. This answer is for informational purposes only and does not constitute the formation of a lawyer-client relationship. Any reader of this answer should not make decisions based upon in without first directly consulting with an attorney