I met again English world

I met English world in junior high school.
I was not very interested in English on my student day.
I don’t understand image of “brother” and “sister”.
I imagined “brother” as older brother, and “sister” as younger sister.
In Japanese, “kyoudai” is separated “ani” into “otouto”.
And, “Simai” is separated “ane” into “imouto”.But, “brother” and “sister” cannot be separated.
I had different answer in my junior student day that I don’t have brothers for my image.
But, I have a young brother actually.
I wonder why these words don’t used each word.
I left from English world at graduate of 2 year Female College.
I came back to learn English for reading English diary book of ALC.
I hope that my English world extend some day.