Disclaimer: The materials provided below are informational and should not be relied upon as legal advice.
If your father was holding the stock only as a trustee for your and your siblings benefits, he should not use the proceeds for purposes other than those designated by the donor (your late grandfather). I suppose if he found out the stock may lose value, depending on the trust terms, he may be able to liquidate the stock and invest them in a safer investment, but only for the purposes designated by the donor. If you are certain your father has misappropriated the funds, you need to immediately consult your own attorney.
It is a challenging and technical issue of fact whether your father was given possession of the stocks or control. If control, there are technical issues of fact as to whether he may have been entitled to exercise certain liquidation and/or re-investment powers. You will need some professional assistance in making the factual determinations necessary to know whether any impropriety occurred. But realize that, even if your father was irresponsible or exceeded his powers with respect to the stocks, unless your father has assets to make good on the losses, there may be no meaningful remedy here.
My responses to questions on Avvo are never intended as legal advice and must not be relied upon as legal advice. I give legal advice only in the course of an attorney-client relationship. Exchange of information through Avvo's Questions forum does not establish an attorney-client relationship with me. That relationship is established only by individual consultation and execution of a written agreement for legal services.
It is very important to know exactly how title to the stock was held to determine your rights. Can you talk to your father (or mother) about the stock? One of the difficulties you may encounter is your age. Once you turn 18 you may want to retain and discuss this issue with an experienced attorney. Prior to 18 you are going to need the assistance of a qualified adult.
No legal representation exists by virtue of this answer. It is recommended that you contact an attorney directly for a more complete response.