All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
We ask Allah to increase you in adherence to the truth and zeal in guarding the sanctities of Allah, and to guide your sister.
Since you have already advised her, you have fulfilled what was required of you. However, it is best that you continue advising her wisely, if you hope that the advice will benefit her, for Allah, the Exalted, says (what means): {So remind, if the reminder should benefit} [Quran 87:9].
Al-Sa'di
said in his tafsir: “So remind [them] with the Sharia of Allah and His signs, if the reminder benefits — meaning: so long as the reminder is accepted and the admonition is heard, whether the reminder achieves the full intended purpose or only part of it. And the implied meaning of the verse is: if the reminder does not benefit — such that the reminding would increase evil or diminish good — then reminding is not commanded in that case, but rather prohibited.” [End of quote]
You should also, in a kind manner, remind your father of his responsibility for this girl [i.e., your sister], and that she is a trust placed in his care, so he must strive for her guidance and prevent her from going out wearing perfume, for Allah, the Exalted, says (what means): {O you who have believed, protect yourselves and your families from a Fire whose fuel is people and stones, over which are [appointed] angels, harsh and severe; they do not disobey Allah in what He commands them, but do what they are commanded} [Quran 66:6]
As for yourself, you must be cautious about mixing with your female cousin, lower your gaze from her, and be wary of being in seclusion with her, for the Prophet (ﷺ) said: “No man should be alone with a woman except that a mahram is with her.” [Al-Bukhari and Muslim]
Allah Knows best.