I have a sister, who, if I were to give her a Hijaab as a present, would wear it by the permission of Allaah. However, she dislikes the Hijaab. How can I tell her that this takes someone out of the fold of Islam?

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

You have to help your sister to wear the Hijab by all possible means and gently encourage her to do this. You should explain the evidence in Islam for the obligation of Hijab to her so that she wears it out of conviction and willingness. As long as she obeys you and wears it if you give it to her, then you should do so. If she wears it, experiences the benefits of it and gets accustomed to it, then she will love it, Allaah willing.

As for your statement "How can I tell her that this takes someone out of the fold of Islam?", if you mean by not wearing Hijab, then, although it is a huge sin and a violation of the instructions of the Book of Allaah Almighty and the Sunnah of His Messenger, sallallaahu ‘alayhi wa sallam, it does not put one beyond the pale of Islam in the sense that she is no longer a Muslim. Committing a sin, even though it is a major one, does not put one beyond the pale of Islam according to Ahl As-Sunnah and the majority of Muslims as long as it does not constitute Shirk (polytheism). However, if you mean that she hates the teachings of Islam or Islam as a whole, then there is no doubt that this is clear disbelief that renders deeds worthless and puts one beyond the pale of Islam. Allaah Almighty says (what means): {That is because they disliked what Allaah revealed, so He rendered worthless their deeds.} [Quran 47: 9] 

You have to explain the gravity of that matter and the necessity of repenting from it to your sister, and you should emphasize that it is a form of disbelief. If she goes back to her religion and the right path, then remind her of the religious obligation of wearing the Hijab and its importance in that time. You should inform her that the whole body of a woman is ‘Awrah (i.e. should not be uncovered).

Allaah Knows best.