Sunni

Yes. This Hadeeth of Ghadeer has been narrated by biggest number of companions who witnessed it and heard it from the Prophet Muhammad (SAWA). It is more than Mutawatir in Sunni books as well as Shia books.

1. Sibt ibn al-Jawazi said in his book Tathkirat al Khawas  (Pg 30 & 31) "All the Ulama of Sirah have unanimously said that the incident of Ghadeer took place when the Prophet (SAWA) returned from Hajjatul Wida when he said in front of 120000 companions : Of whomsoever I am the Maula Ali is the Maula. The full Hadeeth mentions that the Prophet (SAWA) started his sermon "Allah is my Maula and I am the Maula of all Mo'meneen". Then he said am I not more entitled on the Mo'meneen than themselves upon themselves? All said "Yes". He (the Prophet SAWA) : For whomsoever I am his Maula, this Ali is his Maula.

2. Abu Ishaq al Tha'labi mentioned in his Tafseer that when this announcement was spread in different places a man by the name of al Harth ibn al No'man al Fahri came to the Masjid of the Prophet (SAWA) and objected on the announcement of Ghadeer and asked the Prophet (SAWA) : Is it from you or from Allah SWT? The Prophet (SAWA) became angry and his eyes became red and he said : By Allah SWT who is La Ilaha Illahu it his from Allah SWT and not from me.

3. In this Sahih Muslim (Vol 7, Pg 122 & 123) a Hadeeth narrated by Zaid ibn Arqam that the Prophet (SAWA) said in Ghadeer Khum : I am leaving among you two important things ; the first the Book and Allah SWT and my Ahlul Bayt (AS). 

4. Ibn Hajar said : Hadeeth al Ghadeer is definitely authentic and there is no doubt about it. 

5. An Nasai in al Khasa'is narrated from Zaid bin Arqam Hadeeth al-Ghadeer (Pg 39).

6. It was also narrated by al-Mohib al-Tabari in Thakha'ir al-Oqba.

7. Also in ibn al-Maghazili in al-Manaqib (Pg 29).

8. Also al-Muttaqi-al-Hindi in Kanz al-Ommaal (Vol 1 ; Pg 166)

9. Also al-Qurtubi in his Tafseer (Vol 18 ; Pg 287)

10, Also al-Haakim-al-Hasakani in Shawahid-al-Tanzeel (Vol 1 Pg 158)

Many other Sunni books have narrated Hadeeth al-Ghadee with the frase of Allah is my Mawla and I am Mawla of all believers and forwhom soever I am the Mawla, Ali is his Mawla.

Wassalam.

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.