I know many people who have an attitude against prayer. Some say, "I'll pray when I feel ready", or "I will pray on my own intention; I don’t want anyone to tell me when I should start praying. I will feel, spiritually." - I hear the same type of reply with Hijab. How do I tell these people the importance of prayer?
Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
A person is Islamicly qualified to perform acts of righteousness when he reaches the age of puberty provided he is sane. The most important of these acts is performing prayers. Prayers are the best acts of righteousness, and they are the first thing a person is to account for.
Allah threatened of severe punishment the person who neglects prayers, and will resurrect such a person with Pharaoh, Haman and Qarun.
Allah ordained women to wear Hijab. The Hijab makes them pure and chaste. By wearing Hijab, women are saved from the Anger of Allah. The worst of women are those who do not dress modestly.
The Prophet (Sallallahu Alaihi wa Sallam) used to pay homage to women who dressed modestly. So you have to advise your sister and admonish her using texts of revelation that arouse the interest in doing well and in avoiding evil reminding her of texts about the punishment and torture of Allah.
In order to do so, you can read (for more information) the books about the importance of prayer and Hijab: The book entitled 'Riyadh Al-Saliheen', and Al-Targheeb and Al-Tarheeb. Read these two books to her, and a great impact will happen as a result. Insha Allah.
Allah knows best.
You can search for fatwa through many choices