"Many sisters do not sleep."
Translation:Multae sorores non dormiunt.
Adjectives fall into one of two categories: ones with endings generally matching those for first + second declension nouns or ones with endings generally matching those for third declension nouns. They keep those endings no matter what flavor noun they get stuck to. The ending does have to correspond to the noun's number + case + gender, though.
Multus (-a, -um) is an example of the first type. Feminine nouns get the first-declension endings and masculine+neuter nouns get the second-declension ones. E.g. multi viri, multae feminae, multae sorores (nominative), multos fratres (accusative), multa crustula (neuter nominative or accusative plural) etc.
Lots more detail in https://en.wiktionary.org/wiki/multus and https://en.wikipedia.org/wiki/Latin_declension#Adjectives.
TL;DR -- what Moopish wrote while I was writing this.