The stress is on the final syllable as opposed to on the ё like in сёстры.
Сëстры is nominative plural of сестра, but after the number two, genitive singular is called for: сестры, stress on the last syllable
The number 2 forces the genitive singular on the noun it describes. Два is used for masculine and neuter nouns and две for feminine nouns.
две сестры = two sisters (literally "two of a sister") два мальчика = two boys (literally "two of a boy") два окна = two windows (literally "two of a window")