'Other' as an adjective is invariable, the same as all adjectives in English. In French, adjectives change according to gender and number, but in English they don't.
my other sister
my other sisters
my other brother
my other brothers
You see? Singular, plural, masculine, feminine - no change.
'Other' can also be a pronoun. As a pronoun, it has a singular form (other) and a plural form (others).
I don't want this book. I want the other.
I don't want these books. I want the others.
I hope that answers your question.