What is the ruling on attempting to correct the negative impression of religion and Hijab held by a sister, which may have been caused by harsh treatment towards her, and is it permissible to demand that the sister repent from disbelief if her heart has become constricted by religion and Hijab?

We advise you to replace your harshness with gentleness and kindness towards your sister, and to continue admonishing and advising her with wisdom, good counsel, and a gentle approach. Remind her of the virtues of the hijab, that it is a covering, protection, and chastity, and of the danger of removing it, as it exposes her to Allah's wrath. Try to connect her with righteous friends. As for what you mentioned about her hating the hijab and leaving the fold of Islam, that is a whisper from Satan. The default for a Muslim is to remain in Islam, and one does not leave it except with certainty.