First, how to you know this “recent family member” is really a family member?
Second, before entering a new person to your file you should always check to see if they are already part of your file.
What to do.
Before doing anything back up your file.You need to be able to get back to where you are now.
You can look for and merge duplicate individuals.This can get complicated if parents, spouses, and children are also duplicated.
You can link all sources for the unrelated tree to the related tree and copy paste any notes from the unrelated tree to corresponding person in the related tree.Once this is completed you can delete the unrelated tree..
If the unrelated tree is a mix of duplicated people and new people you can detach the new people from the unrelated tree and attach them to the corresponding relationships in the attached tree.