How can my sister reconcile her desire to observe hijab with her husband’s opposition to it?

In 2010 my sister did her nikah with one of my cousins, at that time my sister was not much into deen and did not use to do Hijab, but now after her nikah she started to come closer to deen and now Alhamdulillah, she does Shariah purdah but the problem she faces now is that her husband does not want her to observe Hijab as he is very much into fashion. And before the nikah my sister was also of the same kind, she was too much into fashion that was one of the reasons they both liked each other and married each other. On 2011 Jan 29 in-sha-Allah, the rukhsati ceremony will be done but now my sister fears that if her husband will not allow her to wear hijab afterwards what is she going to do as she cannot let go of the hijab. Now that she knows that it is a fardh for every woman and if she observes hijab then she is not making her husband happy and that is also a major sin. Please help she is very afraid that something bad will happen and break her marriage.