Does anything stop identical twins from traveling on each other's passports?
Technically, yes. It is illegal, that's the one thing that will stop most people.
In practical terms, can identical siblings use each other's credentials in many scenarios, yes, of course, especially if the picture is at least a year old.
The inclusion of multiple authentication factors though, such as fingerprints, make it much more difficult, especially when the intent it to verify identification.
However, keep in mind that verifying fingerprints in questionable situations is not a quick process. The software is very good at matching but an officer may still needed to verify the match. I would not be shocked (meaning I am admittedly speculating) to find out this step was skipped if the initial entry interview raised no suspicions.