In 1966 Murray Bowen, M.D. published Bowen's family systems theory. One of the most critical elements of Bowen's eight part theory was the
concept of triangulation in the family. Simply put, when someone finds themself in conflict with another person they will reach out to a third person. The resulting triangle (e.g., three-person exchange) is more comfortable as the tension is shifted around three people instead of just two.
Triangulation is widely recognized as a stabilizing factor in a family, at work, among social groups, etc. We all engage in triangulation because triangles help us cope when we are struggling with another person.
While triangulation is an important stabilizing factor, at times triangulation can be a seriously destabilizing factor. "Bad triangulation" (i.e., pathological triangulation) can cause more turmoil in a relationship, polarizing communications and causing conflict to escalate.
Here are some ideas on how to work through this:
https://bpdfamily.com/content/karpman-drama-triangle