Sunni

This claim is baseless. Those who claim this false story say that Safawi ruler forced Sunni to follow Shia faith. Millions of Iranians are still Sunnis and no one forced any one to accept Shia faith. Safawis ruled over many countries including Iran, Afghanistan and parts of Indian subcontinent but history never recorded any force on any one in the matter of faith. Till today Sunnis of those areas are still Sunnis. This fact itself is a living evidence against that false claim.

Wassalam.

Writing Hadeeth was officially banned during the governments of Abu Bakr and Umar and Uthman and Bani Omayyah till the time of Umar ibn Abdul Zeez which was on years 99 till 102 after Hijra when he allowed writing Hadeeths and ordered one of Shaikhs working for the Umayyad government ( Ibn Shihab al-Zohri) to write Hadeeths which are accepted by the government that time.

Mussannaf Abdul Razzaq, Mawatta' Malik, al-Imamah wal-Siyasah are among the early books.

Wassalam.

Fadak was mentioned in many authentic narration in Shia and Sunni books. The narrations about Fadak in Shia books are so many that many scholars complied whole books on this matter.

Wassalam.

Main changes in Salaah were done during the rule of Omar ibn Al-Khattab. There are hundreds of narrations even in Sunni books about these changes initiated by Omar. You can see Al-Nass WAL-Ijtihad by Allama Sharafuddin and also Kitab Al-Waa'il which is a Sunni book by Abu 'Oroobah, stating who initiated what among Muslims. 
Wassalam.

It depends on the intellectual state of your sister. Sometime love disables rational thinking when it becomes a blind love. You should pray to Allah to avoid us and our beloved ones blind love. You should ask your sister whether she is ready to think about the future or not ready. She should think about her future after few years from such marriage when love emotions evaporate as usual. What will be the future of her children if any. Will she take the risk of their possible deviation from the way of Ahlul Bayt (AS)?

Simply tell her to study majority of such love marriages between persons from different faith backgrounds and take the lessons from thousands who suffered and still suffering but can not repair the damage.

When there is a possible harm on your or your future children faith or religion, you should avoid such risk.

If the man is open minded and ready to to read and understand and follow the Prophet and his Ahlul Bayt, then the matter is acceptable. Just promises are not enough, but real practical steps are required before the marriage.

 Wassalam.

It is not our job to determine who will enter Paradise or who will not. Allah knows the intentions and circumstances of every creature, and He grants Paradise to whom He wants, according to His Knowledge and Mercy.

No doubt, the authentic Hadeeth is mentioned in both Sunni books and Shia books states that: Who dies with out giving allegiance to the Imam of his time will die a death of Jahilyyah. ( Musnad Ahmad bin Hanbal, v,13, p. 188 and Saheeh Muslim, v.6, p.22 and Kitab Al-Sunnah by ibn Abi Aasim;489 and many other books of Hadeeth.

Wassalam.

Yes it is better to pray with congregation than praying alone even if the congregation is with Sunni Muslims. When you pray in congregation with Non-Shia Muslims, you need to recite Al-Hamd and other Sura yourself but with out clear voice.

 Wassalam.

Yes it is permissible to pray Eid prayers in Sunni mosque. Eid prayer can be performed in congregation ( Jama'ah) as well as individually (Furada). When pray with congregation of Sunni brothers, you need to recite every Sura and Du'a in whispering without clear voice. This is exactly like praying any prayer with Sunni congregation.

Wassalam.

Following Ahlul Bayt is in fact following the real teachings of the Prophet (SAWA). None of the four Sunni sects (Hanafi, Shafi'ee, Maaliki and Hanbali) were existing during the time of the Prophet, while Ahlul Bayt were living with the Prophet and practicing his teachings.

Details of the rules and worship can be easily found in many websites e.g. www.Sistani.org and www.Al-Islam.org and www.shiasearch.org

Wassalam.

Shia Islam means following the teachings of the Prophet Muhammad (SAWA) narrated by the most pious and most knowledgeable who are Ahlul Bayt (AS) who are the Progeny of the Prophet. Allah (SWT) mentioned in Quran the term Shia for the followers of the Prophets. (And from his (Noah's) Shia was Ebrahim) Sura Al-Saaffaat, verse 83. Quran mentioned (in Sura Al-Qasass, verse 15) a follower of the Prophet Moosa disputing with a person from Moosa's enemies and calls the follower of Moosa as from His Shia.

Wassalam.

This question can be from a person who did not understand the difference between Shia Muslims and Sunni Muslims. Shia Muslims believe that the divine leadership of Islam after the Prophet Muhammad (SAWA) is for Imam Ali (AS) and Ahlul Bayt (AS), according to the orders of Allah and the Prophet,  while Sunni Muslims follow Abu Bakr and Umar and Uthman despite claiming that they respect Ahlul Bayt. Most of Sunni Muslims respect Mo'awiyah ibn Abi Sofyan, whose son Yazeed ibn Mo'awiyah killed Imam Hussain (AS) by clear orders from him to his governor of Kufa and his army.

Imam Hussain (AS) never believed in Abu Bakr as Khaleefa nor in Umar and Uthman and never gave them allegiance as Khaleefa.

imam Hussain (AS) stood against and was killed by Yazeed who was appointed by his father Mo'awiyah, who was originally appointed as a ruler on Syria by Umar ibn Al-Khattab.

Claiming that Imam Hussain (AS) was a Sunni is like claiming that Imam Ali (AS) was a follower of Mo'awiyah.

Such claims go against the clear facts and have no value.

Wassalam.