When some individuals begin to develop boundaries, they say, "But my mother (or father, or sister, or brother) is my best friend." They often feel fortunate that, in times of family stress, their best friends are the family in which they were raised. They don't think they need an intimate circle of friends besides their own parents and siblings.
They misunderstand the biblical function of the family. God intended the family to be an incubator in which we grow the maturity, tools, and abilities we need. Once the incubator has done its job, it's supposed to encourage the young adult to leave the nest, connect to the outside world (see Genesis 2:24), and establish a spiritual and emotional family system on one's own. The adult is free to do whatever God has designed for him or her.
Over time, we are to accomplish God's purposes of spreading his love to the world, to make disciples of all the nations (see Matthew 28:19—20). Staying emotionally locked in to the family of origin frustrates this purpose. It's hard to see how we'll change the world when we live on the same street.
No one can become a truly biblical adult without setting some limits, leaving home, and cleaving somewhere else. Otherwise, we never know if we have forged our own values, beliefs, and convictions—our very identity—or if we are mimicking the ideas of our family.
Can family be friends? Absolutely. But if you have never questioned, set boundaries, or experienced conflict with your family members, you may not have an adult-to-adult connection with your family. If you have no other "best friends" than your family, you need to take a close look at those relationships. You may be afraid of separating, individuating, and becoming an autonomous adult.