想問這句文法有錯嗎? Maybe I m not the "BEST BROTHER",but I want you to know that I ll always be your backing no matter when.?

通常表達"無論如何"會說"no matter what"而不會說"no matter when"
"backing"亦不是英語人常用的用詞，但用意還算可明白，較為英語的類以說法可能是：
I ll back you up no matter what.
I ll support you no matter what.
I m always be on your side no matter what.

除了這是"文意"上的詞意(不是英語人的習慣語)外，另亦有些"文法"亦可以考慮是否要修改：

I may not be the "best brother".
Perhaps I m not the "best brother"

I ll 便不應用always...
always便或者不用will tense.
假如確定必然的話可以說
I m always be your...

除了一些標點符號以外,文法正確!

As long as your elder brother then you ought to know I'm always on your side. Yip