The word "Sustre" is an archaic spelling of "sister" that is no longer commonly used. Its pronunciation can be shown through IPA phonetic transcription as /ˈsɪstər/. In this transcription, the "s" sound is shown as /s/, the "i" sound as /ɪ/, the "t" sound as /t/, and the "er" sound as /ər/. The word "Sustre" is an example of how language evolves over time, with old forms sometimes being replaced by new ones.