How do we compel our 18-year-old sister to pray and wear the hijab, considering that she only maintains her prayers on occasions, does not wear modest clothing, and puts on makeup when she goes out, despite our gentle and firm attempts with her, and do we bear sin because of her actions?

If the sister does not maintain prayer and neglects the hijab, she is in great danger. She must be dealt with gently, kindly, and with good admonition, following the example of the Quran and Sunnah. Continue to advise her in a pleasant manner, taking her best interest into account. You should frequently supplicate for her guidance and investigate the reasons for her deviation; it might be due to bad company, in which case some righteous girls should be assigned to her. The default is gentleness, but if it is highly probable that shunning her will benefit her, then she must be shunned. However, if shunning her is feared to increase her transgression, then do not shun her; rather, continue to advise her. If you fulfill your duty of advising and guiding her, then no sin will befall you because of her.