the reason it is ussually not a problem to eat at a Rebbis house even if he has teenage girls is because most probobly nothing would happen seeing as your Rebbe is sittting at the same table as you unless you have no Yirah of your Rebbe- that should be worked on
it is preferable to sit on the same side aof the table as the girls with your sisters in between because if there across from you, youll be looking at them
although i do not Pasken if indeed someones Rav told them it is assur then they cant really have their own opinion beacause its assur
encouraging a possible issur is crazy and before encouraging it ask your Rav what he thinks
theres no being liberal with halacha