So when you said
If he's asking you to "fill in the form",
then even if he's using different words,
your brother is asking you to be the declarant.
If you choose to do this, then what you put on the form
needs to be true and correct according to you.
It's an offence (yes, crime) to do otherwise.
Frankly, if you don't know the difference between being a witness and being a declarant,
then signing things may not be the best choice.
Further, the witness will need to see you sign it.
No eligible witness, acting ethically, will witness something brought to them,
already signed, by a third person.
that isn't actually what you're doing.All I need to do is be a witness
If he's asking you to "fill in the form",
then even if he's using different words,
your brother is asking you to be the declarant.
If you choose to do this, then what you put on the form
needs to be true and correct according to you.
It's an offence (yes, crime) to do otherwise.
Frankly, if you don't know the difference between being a witness and being a declarant,
then signing things may not be the best choice.
Further, the witness will need to see you sign it.
No eligible witness, acting ethically, will witness something brought to them,
already signed, by a third person.