How to say this sentence in Japanese?

The sentence is "The younger brother knew it wasn't the mother". I study Japanese in school, and I need to translate this sentence in Japanese for my project. Feel free to ask clarifying questions.

If you are studying Japanese, then try to construct the sentence yourself. It might not be perfect, but that is part of learning. You can only learn from your own mistakes.

Hi. I'm Japanese teen.
I thought it is
「弟はそれが母親ではないことを知った」.

I'm happy about someone is trying to study Japanese.

Do Japanese people use それ for people?

Original English sentence says 'it' and its literal translation is それ but it seems to be その 女性は, それをしたのは, そこにいたのは etc. depending on what writen before.

To KOHEI-san,

As the original poster said that it was for his learning and his project, the first answer intentionally refrained from giving out the answer, as it is better for learning if the learner gives it a try at writing out what he/she wants to say. You learn from your mistakes. I (I am Japanese) would also be glad to comment and make suggestions for improvement if the original poster came up with his attempt. Thank you for your understanding.

KOHEIさんへ
質問者が、日本語を勉強していて、自分のプロジェクト用にこの文の日本語訳が欲しい、と言っていたので、最初に答えた人は、意図的に、答えを書かずにいたんです。自分が言いたいことを学習者がまずは自力で日本語にしてみることが大事ですから。ミスをしてこそ、学べますから。私も（日本語話者です）、もし質問者が、この文はどう？？と、自分で書いてみた文を投稿してくれたら、喜んで添削するつもりです。意図をご理解いただけたら幸いです。

Do Japanese people use それ for people?

No, but we Japanese people use それ for "it." Even if "it" represents その女性, you can still use それ.

Meanwhile, translations totally depends on the context, so we can't give an accurate translation for the original post, even if it's not for a school project. Personally, I don't agree with Kohei-san's interpretation, but again, that depends on the context of the sentence in issue.