Looks to me like moss you use in creating landscapes for miniatures. Is her brother by any chance into tabletop games like warhammer ?
It's definitely moss. If you're asking why he has it... Who knows? I bet you have a lot of odd shit hidden away too. Maybe you shouldn't be going snooping through other peoples' cupboards.
Because you make little battle maps in games like that, and people use moss for shrubberies on those battle maps. He could also be doing any amount of modelling with it, from war miniatures to train sets.