Why do you care what she looks like? Your brother likes her and when he asked for your opinion it was probably more about her as a person than her as a shag, since I doubt your brother's top requirement for choosing a girlfriend is "will my bro want to fuck her?". You should have stopped at "Yeah she's nice".
You should probably not say anything now. Just let it blow over. If you bring it up again it's going to seem weird.