I think my brother's wife is brainwashing my 21 month old niece.
That seems strange. Do you have a reason for accusing your sister-in-law of brainwashing your niece?
Yes, my little niece is behaving strangely, probably my sister-in-law is giving my niece 'fashion talk'.
It's understandable that you don't agree with the way your niece is being raised. However, you should not lie to your brother just because you don't like your niece being fashion minded. Instead, you should respect the choices that your sister-in-law chooses to make regarding her child's clothes.
I'm going to keep talking to him about it until my brother believes me that his wife is brainwashing my niece.
But you don't know for sure, so accusing your sister-in-law is wrong. You should find out for sure before spreading a lie to your family.