Как ответить на вопрос?  How is your mother

My mother is fine, thank you

My mother is fine, thanks. - самый краткий вариант ответа.
Можно и поподробнее:
My mother is very happy, I bought a pet for her as a B-day present, so she is never bored now. She is interested about your family, call her as soon as you can, please.