Latin and latinized words have no real "set" pronounciation as Latin is a dead language and nobody really knows for sure. When I was taking my biodiversity courses, the instructor even gave us multiple pronounciations for a few different things. And hearing the Russian prof say them? Well, thank God he relied on powerpoints.
I usually hear it like this: "Sin" as in the thing you'll go to hell for, "en" like saying the letter 'N,' and "sis" as in 'sister.' Alternately, some say the first syllable like "sine" as in the sine wave. I don't think it particularly matters either way.