Matching up new computer to old Time Capsule backup
Time Machine backups are per-machine. You can't merge the data from your old MacBook Air with the backup for the new MacBook Pro. (Whilst there are technically methods for doing this they are almost universally a bad idea and carry a significant risk of corrupting the old backup and creating a ton of bloat for any future restore. See: Is there a way to merge Time Machine backups?. 

If you still have the .sparsebundle file for the old MacBook Air and the encryption password (if set). You should archive this (somewhere that isn't the time capsule) so that you can access this data if required. You should then have enough space to instruct your new MacBook Pro to backup to the Time Capsule.

To check the actual backup files you should open the Time Capsule directly in Finder. Based on what you've said thus far I'd expect to see the following:


MacBook Air Sparsebundle
MacBook Pro Sparsebundle
Wife's Mac Sparsebundle


You'll want to copy the MacBook Air Sparsebundle somewhere safe - this is essentially your long-term backup containing all the backup history from your old MacBook Air.