Your sisters are very fortunate to have you for their big sister, Philomene Marie!
One observation I can offer is, every girl on the planet would stop wearing
indecent clothing if the boy she wants attention from would not give it to her
unless she wore some proper clothing. That is to say, if the boys stopped
paying attention to the street-walker look, it would cease overnight.
Therefore, one thing Dad could do is to lavish praise on appropriate dress,
and consistently disapprove of the opposite. Girls want to be noticed by their
father.
As for selecting and obtaining tasteful clothing, that's generally Mom's department.
But Dad can be supportive of her, too. A little encouragement from her
husband goes a long way, because mothers like to be appreciated, too.
Together, Dad, Mom and big sister are fighting an enormous battle in these
days, when even the elect (if possible) would be deceived. This is really
diabolical disorientation, when fashions have been introduced that are not
pleasing to God. Many souls are lost in hell because of sins of the flesh, more
so than for any other reason. And boys and men are constantly at risk for
those kinds of sin when they're around girls and women who dress immodestly.
Does your sister want to be a girl who is the reason that a boy she wants to
be appreciated by commits sin when he looks at her? Doesn't she know that
he is only going to respect her less because of it? If he says otherwise, he's
either fooling himself or else he's just lying to her - usually it's the LATTER!
And then, why would a nice girl want anything to do with a boy like that?