Brothers and sisters are among the relatives with whom Islam commands us to uphold ties.
The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Allaah says: ‘I am al-Rahmaan (the Most Merciful) and this rahm (tie of kinship) has a name that is derived from My name. Whoever uphold it, I will take care of him, and whoever severs it, I will cut him off.’” (Narrated by al-Tirmidhi, 1907; Abu Dawood, 1694; classed as saheeh by Shaykh al-Albaani in al-Silsilah al-Saheehah, 520).
And the Prophet (peace and blessings of Allaah be upon him) said: “Whoever would like his lifespan to be extended and his provision to be increased, let him uphold the ties of kinship.” (Narrated by al-Bukhaari, 1961; Muslim, 2557)
Among the rights that they share with other Muslims, except that their rights are stronger, are: that you should greet them with salaam when you meet them; you should accept their invitation when they invite you; you should say “Yarhamukum Allaah (may Allaah have mercy on you) if they sneeze; you should visit them if they fall sick; you should attend their funeral if they die; you should go along with them if they swear that you should do something; you should advise them sincerely if they seek your advice; you should not backbite about them in their absence; you should love for them what you love for yourself and hate for them what you hate for yourself. All of that is narrated in saheeh ahaadeeth.
Their rights also include: that you should not harm any of them in word or deed. The Prophet (peace and blessings of Allaah be upon him) said: “The Muslim is the one from whose tongue and hand the Muslims are safe.” (Narrated by al-Bukhaari, 10; Muslim, 40). And he (peace and blessings of Allaah be upon him) said, in a lengthy hadeeth in which he enjoined a number of virtues, “… If you cannot do that, then leave people alone (and do not offend them), for that is an act of charity that you do for yourself.” (Narrated by al-Bukhaari, 2382; Muslim, 84).
And Allaah knows best.