I can compare the hijab situation to when my mother divorced my father via khula.

My mother is from a strict conservative family. As she grew up, she was given a strict Islamic upbringing. She always wore a burqa. When she got married to my father, he deceived everyone in her family. He was an atheist. He lied about wanting to revert to Islam (although he attempted 10 years later and still attempts to revert, he still doesn't know a single dua). He used to drink a lot and beat up my mother. He did not allow her to give me and my sister a strict religious upbringing. My mother used to hire a maulvi in secret to teach us when he was not in town. He did not allow my mother or sister to wear a hijab. He still hates it when I say Salam to him or wear a cap, saying that I look like a terrorist. Finally, 17 years after their marriage, my mother couldn't take it anymore when he accused her of theft. She divorced him.

After divorce, my mother returned to her strict religious ways. She wore a burqa for many months. Then, suddenly, she stopped. She wears a scarf while going out now, but an hour later, it drops down to her shoulders. My sister hates wearing a hijab, and rarely prays. She questions Islam a lot, and is almost an agnostic at this point.

I don't know what made your sister in Islam wear a hijab suddenly and discard it. I guess women who dislike wearing it at some point, decided to do so after a break up of some sort. For example, there is a sister in my class who started wearing a hijab after she broke up with her bf, who is a brother. Now, it's back to no hijab.

I personally suggest that they should be allowed to decided for themselves. Usually they hate it when someone interferes in this regard.

You can always show your sister in Islam this nice video by Baba Ali, may Allah (SWT) bless him, start by showing his other videos first as humor, then get to this:

http://ummahfilms.blogspot.com/2006/...-season-2.html