Your tax person should know that a schedule D for a sale of stock owned by you would not be on your parents' tax return.
First of all, are the proceeds less than $1,600? If so, then just file and pay the minimal amount of tax. Not worth doing any of the following. However, if the proceeds are more than that, and the tax is going to be substantial as defined by you, then you have to do the following:
1. Get the 1099B either from the broker or from the IRS. Call the broker, or go to irs.gov to find out how to request the forms. The 1099B info may actually be in the letter(s) you have received.
2. The 1099B will show the proceeds. Find out when the stock was purchased. If the broker doesn't have the buy price, go to Yahoo Finance and find the buy price by looking up the stock and then going into the historical prices.
3. File a tax return with your own Schedule D. My guess is that you will owe little or no tax.
4. If you owe tax, and your capital gains are more than about $1,600, there is a further complication because you are subject to Kiddie Tax, meaning some of your tax is figured on your parent's rate. You would need to get some info from your parents at that point. However, since your parents qualified for EIC, this should not cause a big tax bill.
5. Check with that brokerage firm to see if you have any other assets. Move them into your own account, maybe at another brokerage place.
This isn't really that complicated for a good tax person to sort out for you.
Finally, since you mentioned it, don't get mad at your parents about money. Chances are they bailed out of the stock because of market conditions. Chances are that it is a minimal amount, you just pay the tax of $100 or less, and it's over.