Your brother is the one who gives you sincere advice. أخوك من صدقك النصيحة

 This means advice in matters of religion and worldly affairs, i.e., he is truthful in his counsel. The preposition "fi" (in) is omitted, and the verb is connected. And in some hadiths, it is said, "A man is a mirror to his brother," meaning that if he sees something his brother dislikes, he informs him of it and forbids him from it, and he does not let him act blindly.

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 This meaning is reflected in English proverbs such as:

A true friend tells you the truth.

And closer to this in terms of wisdom:

"Faithful are the wounds of a friend." This means that a friend's honest rebuke and sincere advice are better than the flattery of liars.

This is an old English proverb, taken from the Bible, and is used to describe someone who gives you sincere advice, even if their words are difficult to hear.

Another example is:

"A friend to all is a friend to none." Because a true friend is not someone who always flatters you, but someone who gives you honest advice and corrects your mistakes.

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يعني، النصيحة في أمر الدين والدنيا، أي صدقك في النصيحة، فحذف في وأوصل الفعل. وفي بعض الحديث، الرجل مرآة أخيه، يعني، إذا رأى من ما يكره أخبره به ونهاه عنه، ولا يوطئه العشوة.