How much of your research methodology is based upon social constructs in your family and how your family of origin interacts with other family members? Have you considered that in families you are researching that family members may relationships entirely different from the ones with which you are personally familiar? Some families are very apt to separate geographically once the children are adults. In others, the geographic distance is never great.
Just because it’s how your family always interacted with each other does not mean that it’s the way all the families you may be researching interact with each other.