Family systems therapy is based on the work of Murray Bowen, who developed family systems theory.
The idea is that a family forms its own system. According to the Cambridge dictionary, a system is a set of connected things or devices that operate together. In the case of a family, the individuals are interconnected and work together. This means if a family member is struggling with, for example, drug or alcohol dependency, anger, depression or eating disorders, that struggle is a symptom of the system.
This doesn't mean that other family members are to blame. It is important to remember that there are no good or bad family members in the system. However, there are behaviours, beliefs and values that can lead to unintended consequences.
To understand these unintended consequences better, we can look at the eight concepts of Bowen's Family Systems Theory, which are: differentiation of self, emotional triangle, family projection process, multigenerational transmission process (Legacy Trauma / Legacy Burdens), emotional cut-off, sibling position, societal and nuclear family emotional process.