Family law is generally a matter for the states. Therefore, when looking for primary law, consider the jurisdiction you are interested in. State laws, cases and administrative material can be found on Westlaw, Lexis, or Bloomberg Law.
Valuable information is also available from the judicial and legislative website of each state. Links to each state's official pages can be found here: http://www.usa.gov/Agencies/State-and-Territories.shtml
50 state surveys of family law issues can be found on the ABA Section of Family Law and the Journal: Family Court Review. There may be others collected by other agencies or associations.