All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Allaah The Almighty Says (what means):
· {Indeed, [O Muhammad], you do not guide whom you like, but Allaah guides whom He wills. And He is most knowing of the [rightly] guided.} [Quran 28:56]
· {Upon you is only [the duty of] notification.} [Quran 42:48]
· {And had your Lord willed, those on earth would have believed - all of them entirely. Then, [O Muhammad], would you compel the people in order that they become believers?} [Quran 10:99]
There are also many other verses that prove that Muslims should advise others and call to Allaah, but Allaah guides whom He wills.
Thus, you should keep on advising your sister gently and with wisdom and good speech to adhere to the rulings of the Sharee‘ah. You can treat her well by talking softly to her and giving her gifts as the human soul tends to love those who treat it well and dislike those who treat it badly. Moreover, you should pray to Allaah to guide her. You can also seek help from you other righteous sisters and Mahram (non-marriageable) women in the family so that they would advise her. However, you should not be bothered with what your friends or neighbors would say because this is out of your hands - and caring for the rights of the Creator take precedence to caring for what people say or think.
Moreover, the responsibility of advising and reforming your sister should be shouldered by your father as he should prevent her from doing these matters that contradict the rulings of the Sharee‘ah. Allaah The Almighty 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 Allaah in what He commands them but do what they are commanded.} [Quran 66:6]
Her father can do that by banning her relationships with bad companions and by monitoring her, giving her less money so as to curb her spending on means to perform evil deeds, and employ other means that may be useful in guiding her to the right path so that he would be free from liability on the Day of Judgment. May Allaah guide us to what He likes.
Allaah Knows best.