There is a lot of valuable information contained in a Declaration of Intent. It gives the name of the person, her/his age and the country from which s/he departed. It will give the port and date of entry into the United States. It will also give the name of the ship. If the person is married at the time s/he applied for a Declaration of Intent, the spouse's name and occupation is listed. It will list the children's names and ages.
If your relative was issued a green card you can request a copy of her/his application to register as an alien from the INS. The application contains the same information as the Declaration of Intent. It may not contain the spouse or children.