In the book Aqaed al-Haqqa by Ayatullah Sayed Ali Sadr we find infallibility categorized into 3 levels:
1. Ismat ul Kubra which is reserved for the 14 Infallibles. They do not commit sins, mistakes or even tark al-awla (choosing the less preferred method). We as Shia however do not believe that any of the prophets, from their birth until their death, committed any sins at all whether big or small. We see that the honorable Shia scholar, Sheikh Mufid stated that the prophets and Imams never committed sins or even left the recommended actions (mustahab) and that they were perfect in every aspect of their lives.
Another point to bring is that the prophets are not infallible by force. Rather, they are infallible by utilizing their intellectual ability to not succumb to sin. They have the ability to sin, but they choose not to sin. They know the effects and evil disgusting nature of sin and are repulsed by it. An example would be, if we saw an old piece of food that was rotten would we eat that food? Of course not! It is repulsive to us because we see its rottenness. This is how the prophets see sin. They see the rottenness in disobeying God, whereas most of mankind is attracted to the sins because they have been made fair-seeming to them.
2. Ismat ul-Ithbaatiya which is for the general prophets where they do not commit sins, but it is possible for them to do tark al-awla (choosing the less preferred method).
3. Ismat ul- Thubutiya which is an acquired level of infallibility by abandoning sins and continuously doing their responsibilities to Allah and are obedient to Him. They are just individuals and are far away from deviation. We find this type of acquired infallibility in some of the progeny of the infallibles such as Zaynab, Abbas and Masuma (as) and others.