Translation:Grandfather says that younger sibling's voice is too loud.
While it's common (and more polite) in Indonesian to refer to people by who they are rather than their name, in English that only works with certain words (dad, mom, grandpa, grandma, etc.) but for brothers and sisters you need a pronoun. (Also "little should be accepted as well as "younger".
Grandfather says that my younger brother/sister's voice is too loud. Grandfather says that my little brother/sister's voice is too loud.
'Younger sibling' would not be used in a sentence such as this. As well as needing a pronoun, it should be qualified by stating who the younger sibling is.
E.g. "Grandfather says that his younger brother/sister's voice is too loud", or "Grandfather says that my Great-uncle/aunt's voice is too loud.