Translation:My little sister is very quiet at school.
In this case, で is for place of action, the place being, school, and the action being, behaving quietly. So, if you were to translate literally, it would mean,
My sister, at school, is [ behaving ] very quiet [ ly ]
Was confusing for me as well but your explanation makes it clear. At least until I meet a slightly different variation, and I know I'll make the same mistake again :P
The subject is "little sister" the school is were the action took place
Does anyone have a good mnemonic for "little brother" and "little sister"? I just can't seem to remember them
Is いもうと another one like ちち or はは where you really only say it about your own family members?
I manually typed in "いもうとは学校でとてもしずかです” on PC and it marked it wrong. It doesn't give the option to report my answer as correct either.
You'd use younger sister in this case, young sister is incorrect.
We use the comparison form of young and old when referring to siblings because we are comparing their age to our own and stating that they are in fact younger or older than us.
Let's use the example that a man is 90 and his sister is 80. His sister is old, but in comparison to the man, she is younger than him. You couldn't refer to her as young. But you could still say she's younger than him, or that she's his younger sister.