This comes up for me a lot! The profiles were created by others, but the family group is incomplete in a way that leaves out my ancestor. Or... this is one of those from whom I am descended in more than one way.
There is actually a VERY easy fix.
Check that existing profile and note the WikiTree ID, which will be the last name, a dash, and a number (this is the last part of the web address for that profile). The WikiTree ID you mention, for example, is Stewart-43748 your own is Cox-27995
In the profile, I click on the EDIT tab, then look at the right hand column that lists all the person's family members. Each has options for adding, removing or editing. parents, siblings, spouses, children.
When you click on add a.... one of the first lines of the page that pops up gives you the option of entering a WikiTree ID. Enter that last name-number and click GO.
To add a sibling, the profile has to have at least one parent.
Hope this helps!
Alternatively, you can start adding information as if you were creating a new profile, then choose from the list of potential matches that develops as you key in the data for your relative.
