As-salamu alaykum sister,

Thank you for writing to us. While you could threaten her, you would not like the results. It would only serve to make her want to run further away from the hijab as she may associate it with punishment and manipulation. I would however, encourage her to wear it by offering her small rewards when she does wear it. This way, she will associate it with good things not bad. Also, encourage her to find her own style of hijab which she feels comfortable in. Show her different styles of hijab of girls her age in Islamic magazines and on-line. Ask her about her favorite colors, materials, styles etc. Look at the clothes she wears and notate what she likes. Try to buy scarves that will match or compliment her style and leave them lying out for her. It may pique her interest.
Getting her involved in Islamic social activities with girls her age who wear hijab may also help. As she goes out more often with girls who cover, she may eventually decide to cover all the time. It is something she will need to get use to, to grow into and accept for the sake of Allah, as well as wanting to conform to her peers around her as she is still young. As she is at an age of understanding, it is important to talk about the wearing of hijab and why women are to wear hijab. You can discuss how Allah commands women to cover as He loves us and want us to remain safe, sage as pearls in a shell as diamonds in rocks. You can illustrate and compare her beauty to something that is sacred and to be cherished. Most important is to advise her on obedience to Allah and love for Allah regarding wearing hijab, and ask her to pray on it.
You can’t really force her sister, you can only gently guide her, encourage her, show her the blessings and benefits and provide her with good examples. Hijab is something that not only reverts, but young girls born into Muslim families sometimes have to adjust to. Loving, gentle encouragements will go much further insha’Allah, than harsh strict, punitive measures. You want her to fall in love with hijab, not turn it into an object of resentment and hatred. Insha’Allah with patience, time and encouragement, your daughter will outgrow her fear of what her classmates will say, and be more concerned with pleasing Allah. It is a growing process. At this age it is difficult, but with difficulty comes ease. You are in our prayers, please let us know how things are going.