Use wikitree's "relationship finder" at the bottom of the page..Look for this: "your genealogical relationship with..." - click on the link and enter the Wikitree-# for each person being compared.
If you are interested in finding relationships with living persons. Simply go up a generation or 2 in each person's family tree until you arrive at a deceased person. Then use THESE Wikitree #s for the comparison.
Once the relationship is known (if it exists) you can adjust the level of the relationship to the living persons you're really comparing.