Why is the plural of 'nemesis' "nemeses" and not 'nemesii'?
and how does one pronounce "nemeses" different from "nemesis"?
Why is the plural of 'nemesis' "nemeses" and not 'nemesii'?
and how does one pronounce "nemeses" different from "nemesis"?
Nemesis is directly taken from Latin (which borrowed it from Greek). In Latin, almost all words ending in -is have -es as plural form. You will see the -ii plural form only for nouns which end in -ius.
You can hear the difference between the pronunciation here; nemesis rhymes on this and nemeses rhymes on these.