Praise be to Allah.

Firstly: 

The Muslim woman is obliged to wear shar‘i hijab (hijab as prescribed in Islamic teachings) in obedience to Allah and His Messenger, and so as to protect and maintain her honour, good conduct, religious commitment and chastity. 

Please see the answer to question no. 13998

Secondly: 

The guardian should instruct his daughters to wear hijab and make them adhere to it, and the mother should do likewise, because the parents are responsible for their children, and for raising them properly and ensuring that they adhere to obedience to Allah as much as possible. 

Allah, may He be exalted, says (interpretation of the meaning):

“O you who believe! Ward off yourselves and your families against a Fire (Hell) whose fuel is men and stones”

[al-Tahreem 66:6]. 

Al-Bukhaari (844) and Muslim (3408) narrated from Ibn ‘Umar (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Each of you is a shepherd and each of you is responsible for his flock.” 

The same applies to all members of the family; each of them must help his brother or his sister to obey Allah. 

See also the answer to question no. 93775 

If the daughter refuses to wear hijab, then she should be forced to wear it; it is not permissible to be lenient towards her in that regard. Rather if one of the parents tells the daughter to take off her hijab, it is not permissible for her to obey them in that, because there is no obedience to any created being if it involves disobedience towards the Creator. 

Please see the answer to question no. 20791

The excuse that some people give for abandoning hijab and paying no heed to the command of Allah and the command of His Messenger – “I want to wear it out of conviction, without anybody forcing me to do it” – comes from the prompting and trickery of the Shaytaan, and his confusing the children of Adam. 

Then you should say to her, “Come on, wear it now, out of conviction, and forget about the past when you wore it without conviction. Allah and His Messenger (blessings and peace of Allah be upon him) have enjoined it upon you, so you don’t need anyone to force you. Allah, may He be exalted, says (interpretation of the meaning): ‘But no, by your Lord, they can have no Faith, until they make you (O Muhammad SAW) judge in all disputes between them, and find in themselves no resistance against your decisions, and accept (them) with full submission’ [an-Nisa’ 4:65]; ‘The only saying of the faithful believers, when they are called to Allah (His Words, the Quran) and His Messenger (SAW), to judge between them, is that they say: "We hear and we obey." And such are the prosperous ones (who will live forever in Paradise). And whosoever obeys Allah and His Messenger (SAW), fears Allah, and keeps his duty (to Him), such are the successful ones’ [an-Noor 24:51, 52]. 

“You do not need an interval between wearing it out of conviction and wearing it because you were forced to do so, as you claim. This is not a matter of divorce and marriage, such that you need an ‘iddah between the two!” 

Please see also the answer to question no. 114872

What you have to do is explain your sister’s mistake to her, and exhort her to obey Allah and obey His Messenger, and warn her of His punishment, and the evil consequences of sin for the children of Adam. At the same time, you should be kind to her, and try hard to dispel her doubts, if she has doubts; be patient with her, whilst striving to offer supplication for her, asking Allah to guide her to that which pleases Him, for people’s hearts are between two of the fingers of the Most Merciful – if it be His will to make her steadfast in following His command, He will make her steadfast; if it be His will to cause her to go astray, He will cause her to go astray.