Generally, no test series can give you a clear idea but still your Brother i s performing very well.

I have seen various students getting 590-600 in Aakash Test Series in-spite of this, their score in NEET around 470 - 510.

So I suggest you that do your best (as much as you can) in any test. But don't overthink.

Just keep rectifying your mistakes which you(brother) have made in the previous test before giving another test.