Welcome!
Unfortunately, The Family Lines Graph does not take into consideration that a child in the family may not be by birth.
The Relationship Graph does use a dotted line for non-birth children.
For this to display, the child’s relationship to one or both parents needs to be set to anything other than by Birth in the Family’s Children tab.
There are two addon views that will also display dotted lines for non-birth relationships. The addons Graph View and Family Tree View. Note: the Family Tree View is still experimental and is not yet available through the normal addon process but still doable if you are interested.
The Pedigree View will display dotted lines to the parents from the non-birth child. If the child has more than one set of parents, the first set of parents are shown.