My father passed away in June 2015 and there is a pending wrongful death case which until it goes to trial I don't feel my mother will get any type of closure. With that said she sold their family home and moved in with my sister since he passed but purchased a new home that is being built as we speak. She is now having second thoughts and does not know if she will be OK alone. Could we possibly say that my fathers death caused her too much emotional stress to make rational decisions and get her out of this contract and get her money back.

Bobby1704, please ignore the first response. A moderator will delete it. {Done}

You and your mother should do 3 things:
Consult a lawyer by phone
Talk to the building contractor
Contact a real estate broker (the actual broker, not one of the agents)

The sooner she talks to the contractor, the more hopeful any chance of negotiating a compromise. Stress is not going to get her out of the contract. None of the stress is the contractor's fault. She is not a minor, or declared incompetent at the time she signed.

Her best bet will be to let it be finished and sell it.

Details of the stressors are not all that important. The better (and faster) she is at just asking about options, the better this will work out.

She can try to have a broker sell the house with custom finishes.
She can wait til it's finished, and then put up for sale.
This can work both ways - some people hate unfinished new construction, others love it.
An experienced broker will know how to handle it.
Real estate is in some upheaval at the moment, because mortgage rates went up at least 3/4%

Joy gave you good advice. The decision to enter into a contract to build a home is not the same as an impulse buy of furniture or a car. It is unlikely your mother will get out of the contract due to stress.

I think there may be a good chance to sell while still in construction, especially if its priced to just get her money out.

As Scott noted, houses are different because of costing basis. I'm not sure it falls under duress. Duress exists where a person is coerced by the wrongful conduct or threat of another to enter into a contract. This is apparently not the case. A lot depends on the builder, how far along the construction is and the current market. Although a different circumstance, in 2000, we contracted to build a house on our owned property, gave deposit money and almost immediately found out my wife had Breast Cancer. The builder had already paid out some funds to the log home company. He returned all our money except what he had spent. 2 years later, with everything OK, we went back and started again.