It would be really nice if we had the option to add family members to a source in my source box, instead of individually having to go to each member of the family and adding it
The recent update to the Source Box allows exactly this: you can attach a citation by searching by name, by using the PID, or by choosing the profile from your Recents list. The only trick is that this is only available if you access your source box from your account or from a source document. If you access your box from a profile's source list, you can only attach citations to that profile.
To access your source box from your account, click your account name at the top right corner of any "regular" FamilySearch page (that is, any place on FS that isn't this Community), and choose Source Box.
You can also access your source box if you're looking at a record image, by clicking the Source Box button near the top right of the image and choosing Go to My Source Box.
Once you're in your Source Box, click the "Attach" button to the right of a citation to attach it by name, ID, or Recents.