Slavery and families just don't go hand-in-hand. See, slaveowners weren't in the business of helping slave parents and kids stay together unless it somehow benefited the slaveowners, too. Beloved is full of broken families, orphans, and dysfunctional relationships. Do the slaves form new kinds of families? Or are they left to fend for themselves?
Sethe's sacrifice was just an attempt to protect her family.
In Beloved, family has nothing to do with blood relations.