The bad effects of not observing hijab and dangerous results of co-education are before your eyes. Along with the practice of salah and fasting, it is must to make true tauba from these sins, as well as it is necessary to save your family members from it. In order to bring the girl whose condition you wrote to right path, making earnest dua before Almighty Allah by doing tauba and instighfar may be helpful. All the family members should keep on reciting the following dua regularly.
يا مقلب القلوب صرف قلوبنا إلى طاعتك
And try to take her to religious gatherings gradually.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband