Hi Emily,
Yes you can handle the scenario and you can use the UI to do it.
You can create a site based on the sandbox repository of another site. The UI doesn't specifically make this clear (perhaps we should) but you can use the Create Site from Remote option of Create Site to set this up.
For the remote repository: Enter the fully qualified path to the parent site sandbox:
/Users/rdanner/Desktop/crafter/data/repos/sites/MYPARENT_SITE/sandbox
For credentials: select none
for local branch: Set it to the name of your campaign
Staging waypoint is a completely different capability that has nothing to do with branching a site. The Staging waypoint is a publishing feature that lets you publish to a "beta" site before publishing to live. Some people need a external review site in the workflow before going live.