Is one who forges a dependency certificate to obtain half of the deceased's salary committing a sin, and are they entitled to that against my will?

If the salary is allocated to you by the state, and your husband's family, through forgery, caused you to be deprived of it, then it is not permissible for them to take anything from your right except what you permit. This ruling is based on the premise that the salary is allocated solely to you according to the state's system, but the matter might be otherwise. In case of dispute, reference should be made to the law regulating such matters in your country.