Both .is2 and .sit files are special infrared images saved using the IRT Cronista tool for infrared imaging tasks. It is extremely unlikely these files can be opened or saved in another tool, which means there are no alternatives how to achieve is2 to sit conversion.
We have not found any relevant data about this export though, so if you struggle with it, it might be best to reach out to IRT Cronista support or some online user community for help.
Updated: October 19, 2023