Do shareholders have any legal responsibility to one another?
In a large public company the answer is typically no. A large corporation with thousands of shareholders would not have duty between those shareholders, but in a small closely held company, a majority shareholder, that is someone who owns 51% or more of the company, does have a fiduciary duty to the minority shareholders. That fiduciary duty is to ensure that the majority owner does not use his shares to the detriment of the minority shareholder. You should consult with a commercial litigation attorney to evaluate what rights you have.