The concept of mourning for Imam Hussein is among the furu’ or branches of the Shia school of thought and there are many reasons that support it. But regarding the way of mourning one must say that as long as it does not contradict a principle or a ruling that is mentioned in a hadith or Quranic verse, it is allowed and therefore people with different cultures and customs can perform the mourning in various fashions.
It is noteworthy that according to many Ahadith and the Hadith of Thaqalain, the traditions of the Imams are also a source of Islamic law along with the Holy Quran and from the Shia point of view, the sayings of the Ahlul-Bayt are just as authoritative and credible as the Ahadith of the prophet of Islam, therefore if a hadith mentions a tradition from one Imam in this regard, such an act will be permissible.
In the "Detailed Answer" we have explained some of the reasons that prove sina zani to be permissible.
Before answering the abovementioned question we must shed light on a few points:
A: According to many Ahadith and the Hadith of Thaqalain, the tradition of the Imams (which is comprised of the sayings, actions and the taqrir of the Imam) are also a source of Islamic law along with the Holy Quran.
From the Shia point of view the sayings of the Ahlul-Bayt are just as authoritative and credible as the Ahadith of the prophet; therefore if a hadith mentions a tradition from an Imam in this regard, their will be no doubt that such an act is permissible.
B: The concept of mourning for Imam Hussein is among the Foru’uddin or branches of the Shia school of thought and there are many reasons that support it. But regarding the way of mourning one must say that as long as it does not contradict a principle or a ruling that is mentioned in a hadith or Quranic verse, it is allowed and therefore people with different cultures and customs can perform the mourning in various fashions.
We believe that there is much proof that mourning and sina zani for Imam Hussein is allowed and permissible. Among the many Ahadith we will only mention two which explain the traditions of the Ahlul-Bayt in this regard:
1- Imam Sajjad says: "On the night prior to the day in which my father was martyred (Ashura), I was sitting in the tent and my aunt (Lady Zaynab) was taking care of me (because of my illness). My father went to his tent where Jon, the servant of Abuzar was repairing my father's sword and my father would say: "Woe to you O world! You wipe out many of the followers and friends at every morning and night and you will not be pleased to take anything instead of them, Yes, everyone (their lives) are in the hands and power of Allah, and every living being will take the path that I have (and they will all die). He repeated this point two or three times. When I was hearing these poems I understood what the Imam meant and I wanted to cry but I resisted crying and calmed down and at that point I knew that tragedy had descended upon us. But once my aunt Zaynab heard these sentences she could not resist crying because of her feminine sensitivity and burst into tears. She left the tent in shock running towards the Imam saying: "I wish I had died, today my mother Fatimah, my father Ali and my brother Hasan have died, O reminder of the ones who have passed away!, O guardian of the remaining ones!." Imam Hussein looked at his noble sister and said: "O sister, try not to let Satan gain control over you and take away your patience. If they had left the bird of Ghata to itself it would have calmly slept". My aunt Zaynab said: "My brother has prepared himself for death!! Your readiness for death has hurt my heart and has taken away my patience". She then slapped herself in the face and rended her collar and fainted and fell to the ground. The Imam came closer to his sister and sprinkled water in her face and helped her regain consciousness. Then the Imam said: "O sister calm down and be God wary, be pleased of his will and know that the earthly creatures die and the creatures of the skies will not remain and everything but Allah will be destroyed and no one but Allah, the one with no partner, which has created all beings and will raise people (on the Day of Judgment), will remain alive."
There are two ways to prove that sine zani is permissible through this hadith:
A: This hadith shows that the Imam has only advised his sister to be patient when she slapped herself in the face, and the Imam never forbade her from doing so (especially from slapping herself), knowing that if it was a prohibited and Haram act then the Imam would have definitely informed her.
B: Knowing the attributes and qualities of Lady Zaynab which Imam Sajjad says about her: "All praise is due to Allah that you know the truths and possess the knowledge of the different states of people of the earth, and you do not need to be taught or helped to understand." It is very unlikely that she would commit a sin openly in front of others, like her brother.
2- Musafir, the servant of Imam Kazem recounts: "When they wanted to take Imam Musa Bin Ja'far away, he commanded Imam Reza to sleep in his house until he heard news (of Imam Kazem's martyrdom). We would make his bed ready every night in the hallway. After Isha'a Prayer he would go to bed and after Morning Prayer he would go to his own house, and this continued for 4 years. One night that we made the bed ready, Imam Reza did not come to sleep, we and the Imam's family were very worried about the Imam's absence. The next morning the Imam came to the haram (inner section) of the house and told Umm Ahmad: Give me what my father had entrusted you with.", then Umm Ahmad wept and hit her face and rended her collar and said: "By Allah my master has died!" The Imam held her and said: "Do not say anything about this incident and do not show your sorrow and sadness until the governor is officially informed of my father's death". Then she gave the Imam a basket which had the trusts in it along with two or four thousand dinars.
This hadith which has been narrated in the authentic Book of Kafi also indicates that sina zani is allowed, because evidently the imam could have forbidden her from doing so, but didn’t, and this shows that what she did wasn’t haram.
Therefore there is no doubt that the mourning that is practiced nowadays by people that have love and affection for the Ahlul-Bayt and Imam Hussein is allowed, of course if it is done in the correct manner and does not go beyond the limits.
In order to find answers to other questions related to the abovementioned question you can refer to:
Mourning for Imam Hussein, What Keeps Society Alive, Question 348 (website: 352)
The role of Mourning for Imam Hussein, Question 2302 (website: 2518).
 Tagrir of a Ma'asoom means that if someone said or did an act in the presence of the Imam, but the Imam did not forbid him, this shows that it was not forbidden and haram.
 In this regard, refer to Question 6924 (website: 7028) of this website.
 یَا دَهْرُ أُفٍّ لَکَ مِنْ خَلِیلٍ
کَمْ لَکَ بِالْإِشْرَاقِ وَ الْأَصِیلِ
مِنْ صَاحِبٍ وَ طَالِبٍ قَتِیلٍ
وَ الدَّهْرُ لَا یَقْنَعُ بِالْبَدِیلِ
وَ إِنَّمَا الْأَمْرُ إِلَى الْجَلِیلِ
َ کُلُّ حَیٍّ سَالِکٌ سَبِیلِی
 Mufid, Muhammad, Irshad, Sa’edi Khorasani, Mohammad Baqir, pp. 444 and 445.
 "و أنت بحمد الله عالمة غیر معلمة فهمة غیر مفهمة"،
 Al-Kafi, vol. 1, pp. 381-382, with help from the translation of the biography of Imam Musa Kazem as, pp. 226-227.