This is the 16654th most frequent Russian word.
сестер
"Сестер" is the genitive plural form of "сестра," meaning "of (the) sisters."
In this example, 'сестер' is used in the genitive plural form to express absence, indicating he has no sisters.
У него нет сестер.
He has no sisters.
In this sentence, 'сестер' is the genitive plural form of the noun 'сестра' (sister), used here to indicate possession of two sisters.
Я видел двух твоих сестер вчера.
I saw your two sisters yesterday.
Here, 'сестрами' is the instrumental plural form, used because the action pertains to interacting with the sisters through meeting.