fraud?

Extended family members all received similar letters asking them to sign an agreement, to assign and agree to transfer 25% of their interest in assets supposedly discovered (in Germany) by an international search corp. Aren't all of these people entitled to their share without giving 25% of each of their shares to this company? This company is obviously aware of all the heirs, so if one of these heirs has agreed to retain this company to uncover assets, acquire necessary documents and records, and hire an attorney to represent both the heir and the search company in court, then wouldn't all of the heirs still be entitled, even without retaining this company, or would all of the assets go only to that one who signed this agreement?

They would yes if you can be made aware of the claims and pursue them in court or by law. They cannot take away your rights.