lol that is absolutely bizarre.
Does anyone in your house ever do laundry at a laundromat instead of at your place? Or if you live in an apartment do you have a washing machine in your place or do you use one for "all" the people in the complex? Because if one these, someone may have left their bra in the machine by mistake and one of you may have mistakenly picked up the bra with your / their own clothes.
Other than that, maybe your brother's lying, or maybe that bra's been there so long undetected it belongs to one of your brother's ex's.