Growing up, I was convinced my older sister was the favorite child, and so my communication with her at times became argumentative, or even hostile.
Family systems theorists warn of the spillover effect, where parent-child conflict can spill over into other relationships.
How parents communicate with each other and with their children matters, because it can impact how children learn and normalize behaviors such as conflict management. Children learn how to handle conflict by how they see their parents deal with conflict. If parents use dysfunctional conflict tactics with one another, their children are likely to exhibit dysfunctional conflict with their siblings, or even classmates by adopting the same style.
And much like my own story, if parents use different conflict approaches to discipline their children, perceived favoritism can affect conflict between siblings.