You should see a tax professional immediately, and then you probably need to see the attorneys who are handling the closing. It is very likely that you will owe capital gains tax on 1/3 of the gain if the house is sold in its present condition. However, this is not 100% certain because it depends on how your parents gave you the property in the first place. It is also very likely that you can give your share of the house back to your parents before they sell it, and avoid paying any tax. The gift would probably have to be reported, but no gift tax will be owed unless your lifetime total of all gifts and your estate is more than $11 million.
Then, once you have extracted yourself from this situation, see an estate planner, usually a law firm with financial experts on board, before you make any future moves. Depending on exactly what you are trying to accomplish (avoiding probate or protecting the house as an asset in case of long-term medical costs) there are ways to do it that do not put you at extra tax risk.
