Duties and rights of a man towards his parents, wife and children are specifically spelled out in the Qur'an, Sunnah and Ahadith. Are there such specific instructions regarding spinster sisters in the absence of parents? The social norms and the call of conscience are well understood and appreciated.
Human beings have acknowledged rights of the brothers and sisters from the inception of human society. Islamic religious sources have taken this responsibility for granted and the Holy Qur'an only referred to this in the following words:
يَا أَيُّهَا النَّاسُ اتَّقُواْ رَبَّكُمُ الَّذِي خَلَقَكُم مِّن نَّفْسٍ وَاحِدَةٍ وَخَلَقَ مِنْهَا زَوْجَهَا وَبَثَّ مِنْهُمَا رِجَالاً كَثِيراً وَنِسَاء وَاتَّقُواْ اللّهَ الَّذِي تَسَاءلُونَ بِهِ وَالأَرْحَامَ إِنَّ اللّهَ كَانَ عَلَيْكُمْ رَقِيبًا
O mankind! Be careful of your duty to your Lord Who created you from a single soul and from it created its mate and from them twain hath spread abroad a multitude of men and women. Be careful of your duty toward Allah through whom you seek help of one another, and toward the relations. Lo! Allah is constantly watching you. [4:1]
The matters regarding which there exists enough guidance in the human intellect have not been discussed in the Holy Qur'an. However, an individual or the society deviated from the true path regarding such issues during the time the Holy Qur'an was being revealed the Almighty explicated that for the benefit of the believers. Since there was no such case regarding the issue at hand no specific guidance was provided by the Qur'an and the Sunnah. Therefore, we need to pay heed to the inherent urge to protect and show a sense of responsibility toward our spinster sisters as the Almighty has exhorted us upon giving due respect to the relations.