I suspect you’ve basically got it. Polish -cki marks a locative surname (feminine -cka), so Śniadecki means “Of Śniadka”, and your sister would be Śniadecka. Pronounced something like /ɕnja.tet͡ɕ.ki/ “Shnya-detch-ki” (find an actual Polish person for actual pronunciation).
Which village of Śniadka is a matter for following the genealogy.
Śniadka is itself a diminutive of śniady, which is an adjective meaning “tawny, swarthy”, from a Proto-Slavic root meaning “brown”, itself probably derived from a word meaning “copper”.
