This is the 13734th most frequent Russian word.
несовершеннолетней
"Несовершеннолетней" means "minor" or "underage" (referring to someone who has not reached the age of legal adulthood).
The word 'несовершеннолетней' is used as an adjective describing the subject 'она', indicating her status as being underage or legally minor.
Она была несовершеннолетней и не могла подписать документы.
She was underage and could not sign the documents.
The word 'несовершеннолетней' functions as a noun referring to a minor, indicating that the prohibition applies specifically to individuals who are underage.
Несовершеннолетней запрещено посещать клубы после полуночи.
It is prohibited for a minor to visit clubs after midnight.
In this sentence, 'несовершеннолетней' is used as an adjectival form describing the subject 'сестра', pointing out her need for legal care due to her minor status.