The parents of countless self-directed learners (whether they learn primarily at home or at a democratic school) report that their relationships with their children, and the relationships among siblings, are vastly superior to what they previously experienced, or what they see and hear about from their school-entrenched peers. Obviously, this is no panacea for serious issues such as dysfunctional marriages and bad parenting. But the mere absence of battles over homework assignments, getting to school on time, grades, and the myriad other aspects of forced schooling against which young people rebel is a huge contributor to maintaining peace in the home.
The nasty competitiveness between children in different grades (leading many siblings to play one-upmanship with younger brothers or sisters) tends to be replaced by the pleasant (if not joyful!) collaboration between siblings who now can work/play together on shared interests, learn how to get along with others with seemingly conflicting interests in a democratic school or resource center (or in an adult workplace), and then bring those skills home for the enjoyment of all.
It all begins with how adults treat children: This sets the standard for the youths to emulate. Nagging begets whining, and constant criticism begets resentment and withdrawal. When parents are viewed by their children as true allies, rather than as co-enforcers, the family is much more likely to be a happier and more productive system.