The Shia and Sunni Brotherhood

Sayyid Abul Qasim al-Khoei in Miṣbāḥ al-Fuquhā, Volume 1, page 324 says:
"It is a matter of basic common sense that there is no brotherhood or support/partnership between us Shias and the non-Shias."

we read in Mustanad al-Shī‘a,Volume 14, Page 163:
Mullah Ahmad Al-Naraqi says:
"The attribution of belief and brotherhood towards non Shias is completely invalid, and this is evidenced by mass transmitted hadith from the Imams (a.s) mocking, cursing and doing Takfir of the non Shias, as well as calling them worse than Jews and Christians, and filthier than dogs."

We read in Rijaal al-Kashi, Page 204-205:
Sad b. Jannah narrated to me. He said: Ali bin Muhammad bin Yazid al-Qummi narrated to me from Ahmad bin Muhammad bin Isa from al-Husain bin Said from Fadala bin Ayyub from al-Husain bin Uthman al-Ruwasi from Sadir who said:
I entered upon Abu Jafar al-Baqir (a,s) and with me was Salma b. Kuhayl and Abu al-Miqadam al-Haddad and Salim b. Abu Hafsa and Kathir al-Nawa and a group from them and with Abu Jafar (a.s) was his brother Zayd b. Ali.
So they said to Abu Jafar, "We belief in the Mastership (Wilayah) of Ali (a.s), Hasan (a.s), and Husain (a.s) and dissociate ourselves (Tabarrah) from their enemies!"
The Imam said, "Yes".
They then said, "We believe in the Mastership (Wilayah) of Abu Bakr and Umar and we dissociate ourselves from their enemies!"
Then Zayd bin Ali turned to them and said to them, "Do you dissociate yourselves from Fatima (SA)?! You have mutilated/cutoff our affair! May Allah mutilate you! So from that day they were named the "Batriyya"

Imam Roohullah Khomeini in Al-Makasib Al-Moharramah, Vol 1, Page 377-378 says:

فقوله تعالى: (إنما المؤمنون إخوة) (۳) هو جعل الأخوة بين المؤمنين الواقعيين، غاية الأمر أن في زمان رسول الله ﷺ كان غير المنافق مؤمناً واقعاً، لإيمانه بالله ورسوله وبعد ذلك كان المؤمن الواقعي من قبل الولاية وصدقها أيضاً.
فيكون خطاب يا أيها المؤمنون متوجها إلى المؤمنين الواقعيين وإن اختلفت أركانه بحسب الأزمان، من غير أن يكون الخطاب من أول الأمر متوجهاً إلى الشيعة حتى يستبعد سيما إذا كان المراد بالمؤمن الشيعة الإمامية الاثني عشرية.
وأما الأخبار فما اشتملت على المؤمن فكذلك.
وما اشتملت على الأخ لا تشملهم أيضاً لعدم الأخوة بيننا وبينهم بعد وجوب البراءة عنهم وعن مذهبهم وعن أئمتهم، كما تدل عليه الأخبار واقتضته أصول المذهب ).

Translation:
Allah’s words “The believers are but brothers” (Qur’an 49:10) refers to true believers. During the time of the Prophet ﷺ, anyone who was not a hypocrite was considered a true believer because they had faith in Allah and His Messenger. But after that, true faith also required belief in and acceptance of wilayah (the divinely appointed leadership).

So when the Qur’an says “O you who believe”, the address is always directed to true believers, although the conditions of being a true believer have differed in different times. This means the verse was not originally directed only to the Shi‘a, which some might wrongly assume, especially if by “believer” one means specifically the Twelver Shi‘a.

As for the narrations: whenever they mention “the believer,” they mean the true believer as explained above. And whenever they speak about “brotherhood,” it does not apply to those outside, since there can be no brotherhood with them after the obligation of barā’ah (disassociation) from them, their beliefs, and their leaders. This is supported by narrations and is required by the principles of the madhhab.