What is the optimal way to deal with the actions of sisters who do not adhere to the Sharia-compliant regulations regarding dress, perfume, and ostentatious display, in light of the parents' leniency and their reactions, which the questioner deems ineffective?

A father must prevent his daughters from displaying their adornments (tabarruj), as he is the primary guardian responsible for those under his care. He must teach them matters of their religion and forbid them from what is impermissible for them. You should explain this gently to your parents and strive to advise your sisters and work towards their reform. There is no blame upon you after you have made the clarification, enjoined what is good, and forbidden what is evil.