Compiling the family tree of a DNA match to determine the relationship they have with you is necessary when the match is one in which you, for one reason of another, have an interest. Just make certain you are taking your time and compiling the tree as accurately as you can–especially in terms of the biological relationships.
Relying too much on one type of source (particularly obituaries) can increase the probability that compiled tree you create has non-biological relationships in it. Obituaries and some other newspaper social announcements may indicate the relationship between two people is a parent-child relationship when it fact it is not. The most frequent relationships that falls into this category is a step-parent relationship.
If you’ve got the child’s step-father in your tree as their father, it could explain why the DNA match makes no sense. Make certain you’ve looked at as many newspaper references as you can–particularly ones early in the life of the child. If the family was living during a time when census records are public, view those materials as well. Look at obituaries of all grandparents to see how grandchildren are listed–or if some that should be listed are missing. That could be a clue the child’s relationship to the parent was not biological.
The thing to remember about DNA is that it only tells you who reproduced with whom. It does not tell you who actually raised that person or was influential in their life.