It is indeed correct as per the verse (Surah Al-Isra 17:32) you referenced that Allah forbids us from approaching zina (unlawful sexual intercourse), and that zina is an immoral act. It is also correct that in Sahih al-Bukhari » Book of Divine Will » Hadith 6612, the Prophet ﷺ said that the eye gets its share of zina by looking at something sinful, and that the tong gets its share of zina through uttering unlawful words. However, the same hadith states that the private parts turn that into reality (i.e., committing real zina) or refrain from submitting to the temptation.
What you are inquiring about does not reach the level of zina punishable by hadd. Al-Mubdi' (Arabic: المبدع في شرح المقنع) by Ibn Al-Muflih (Arabic: ابن المفلح) states that from a Hanbali point of view:
ولا يجب الحد إلا بشروط ثلاثة ـ أحدها : أن يطأ في الفرج سواء كان قبلا أو دبرا ـ وأقل ذلك تغييب الحشفة في الفرج ـ فإن وطئ دون الفرج أو أتت المرأة المرأة فلا حد عليهما
— NOTE: My own translation, so treat with care:
And the hadd [punishment for zina] is not mandated except under three conditions, out of which one is for [the male] to insert in the opening front or back, and the minimum is that the male's organ glans to disappear inside the female's opening. If less than that, or if it is two females, there is no hadd.
Based on your question, the act of "committed zina by speaking and an looking at a young man" is not considered by Islamic jurisprudence as a punishable act of zina.
You mentioned that you need to do research before making any decision, here are some points to consider:
- There is no special accommodation for disabled persons that I have come across in the matter of willful zina.
- The relatives should assist the person in question not commit further sins.
- The punishment by hadd is the responsibility of the ruler, not to be carried out by common people or relatives.
Your role, at this stage, with your sister is to help her repent by regretting what she had and absolving not to repeat her actions again. When asked about a married woman who got into a relationship with another man and their relationship became known, Ibn Taymiyyah in his book Al-Fatawa Al-Kubra said:
الواجب على أولادها وعصبتها أن يمنعوها من المحرمات فإن لم تمتنع إلا بالحبس حبسوها وإن احتاجت إلى القيد قيدوها وما ينبغي للولد أن يضرب أمه وأما برها فليس لهم أن يمنعوها برها ولا يجوز لهم مقاطعتها بحيث تتمكن بذلك من السوء بل يمنعوها بحسب قدرتهم وإن احتاجت إلى رزق وكسوة رزقوها وكسوها ولا يجوز لهم إقامة الحد عليها بقتل ولا غيره وعليهم الاثم في ذلك
It is obligatory for her children and her responsible family members to prevent her from forbidden acts. If she does not refrain from her sins except through imprisoning her, they can. If it is required to have her bound, then they can. It is not permissible for her children to beat their mother, to continue to treat her properly and rightly, and not to her. They can stop her from committing sin to the best of their ability. If she needs provisions or clothes, they must provide these for her, and they are not permitted to fulfill hadd on her, whether that be killing or otherwise, and they would bear the sin of any such punishment.
No one, apart from the ruler, has the right to impose any form of punishment on your sister as mentioned in Tafsir Al-Qurtubi (Arabic: تفسير القرطبي):
الرابعة: لا خلاف أن القصاص في القتل لا يقيمه إلا أولوا الأمر، فرض عليهم النهوض بالقصاص وإقامة الحدود وغير ذلك ; لأن الله سبحانه خاطب جميع المؤمنين بالقصاص، ثم لا يتهيأ للمؤمنين جميعا أن يجتمعوا على القصاص، فأقاموا السلطان مقام أنفسهم في إقامة القصاص وغيره من الحدود، وليس القصاص بلازم إنما اللازم ألا يتجاوز القصاص وغيره من الحدود إلى الاعتداء، فأما إذا وقع الرضا بدون القصاص من دية أو عفو فذلك مباح، على ما يأتي بيانه
There is no disagreement that retaliation in killing is only adjudged by those in authority. It is the ruler who imposes on them retaliation, establishes the hudud and other things because, although Allah Almighty gives all believers the possibility of retaliation, not all believers are ready to agree to it. Therefore the ruler represents them in the matter of carrying out retaliation and imposing other hudud. Retaliation itself is not obligatory. What is obligatory is not to exceed in retaliation or other hudud. If there is consent to forgo retaliation and to accept blood money or to pardon, that is permitted.
Showing understanding and lending your sister a helping hand is the best approach. Also keeping such matters between you and your sister is best. Leaking this out can only cause more problems. In Sahih Muslim » The Book of Legal Punishments » Hadith 1695 a, when Ma'iz ibn Malik went to the Prophet ﷺ to purify him (by hadd) after committing zina, the Prophet ﷺ first ordered him go back and ask forgiveness of Allah and turn to Him in repentance. It was after Ma'iz insisted more than once to be punished that the Prophet ﷺ ended up executing the hadd. The woman was given time to deliver her baby before the execution of the hadd.