Several years ago in Italy, where "What were you wearing?" is not only admissable, but the first thing rape victims are asked before the Carabinieri will even open an investigation, a judge acquitted a rapist because the victim had been wearing jeans when he attacked her. Of course, everybody knows that you cannot be raped if you are wearing jeans.
This caused a major outcry and every single female Minister of Parliament wore jeans every day to work in protest for almost six months until a higher court reversed this judge's ruling.
In most states, the rape victim's attire is inadmissable as evidence. No woman asks to be sexually assaulted or harrassed, regardless of what she was (or was not) wearing at the time. Make it clear to your sister that this comment was as inappropriate as that brother's behaviour.