This is a good question. This largely depends on the person and family. If the family is a Christian family, then the Bible would be better. This might sound odd to a largely secular audience, but we need to remember that generations of people modeled their lives on the Bible. Moreover, the Bible offers a lot of insights on family relationships. For example, the family is addressed in Proverbs, Ephesians, and several other places in the Bible.
If the family is not religion, insights for sociology would be helpful - especially studies that deal with family issues. Sociology can be very insightful.
In the end, one would not need to make a decision between the two. In fact, hopefully a person can draw upon all material to help with the betterment of family relations.
The bible is always the better choice but you have to be able to understand and have a foundation before involving yourself in it. The bible is the best source for anything in life, it speaks about the different struggle you endure in life but it give you a response and a path to take