Welcome
Probably the best way is to create a set of filters to select your family, tag them, then decide what you will do with any people not tagged.
In the People Filter editor (in a person view, menu >> Edit >> Ferson Filter Editor) create a set of filters.
Filter 1
Ancestral filters > Ancestors of <person>
Filter 2
Descendant filters > Descendants of <filter 1> match
General filters > People matching the <filter 1>
Set the Options
At least one rule must apply.
Note: adding the first filter back into this filter ensures that the endline ancestors are included. They are not their own descendants.
In the People or Grouped People view you can run Filter 2 for your direct relatives but note, this does not include spouses. To add these people…
Filter 3
Family filters > Spouses of <filter 2> match
Filter 4
General filters > People matching the <filter 2>
General filters > People matching the <filter 3>
Set the Options
At least one rule must apply.
Runing filter 4 will select for the most basic of people. But filter 4 can be expanded by adding any or all of these rules to add parents, siblings, and other marriages/children of the spouses.
Family filters > Parents of <filter 3> match
Family filters > Siblings of <filter 3> match
Family filters > Spouses of <filter 3> match
Family filters > Children of <filter 3> match
Once you have the related (and in-law) people you can select all the people and Create/set a Tag.
Now you can create another filter to find those people not related.
Filter
General filters > People with the <tag>
Options
Check the box Return values that do not match the filter rules.