Walter Toman, Ph.D., developed a sibling profile for each position in a well functioning family. Dr. Bowen used the fact-based profiles developed by Toman to develop his own concept of sibling position based on the scale of differentiation.
Toman developed guideline for how the average person, all things being equal, would fit into a family. The sibling position of oldest, middle and youngest would prepare the individuals to relate more or less easily with others. His way of sorting out how people fit with others was to look at the family as niche. Roles develop that ere good fits or poor fits. There were role conflicts and role identifications based on how people were raised and whom they then marry.
If two oldest married then there would be a role conflict. If an oldest man, who only had younger brothers marries a woman who only had a younger sister they would both find they were in unfamiliar circumstances. Whereas if an oldest brother of a sister marries a younger sister of a brother they will be able to identify and find comfort in their positions in relationship to one another. All of this would be influenced to some extent by the past generations sibling positions.
One example of the interacting variables in sibling position would be that an oldest whose parents are youngest, are going to have different kinds of challenges from an oldest whose parents are oldest. People often learn from identification. Those who have to learn from "what not to do", may find their lessons more challenging and therefore take more time to assimilate.
In addition an oldest that grows up in a harsh environment is going to be different from one who has grown up in good times. Sibling position gives us clues to the processes that emerge in a family and it gives us a background to look for the exceptions when the general tendencies do not emerge.
Whenever one has to look at multiple factors there are no simple answers. Cause and effect thinking shows us only how part of the system works. System's thinking looks at many variables in how nature and nurture work and therefore the system as a whole is a complex non linear system.