Do Islamist consider other branches of Islam as their brothers?

The believers are nothing else than brothers (in Islamic religion). So make reconciliation between your brothers, and fear Allah, that you may receive mercy. (سورة الحجرات, Al-Hujuraat, Chapter 49, Verse 10).

But, Ahmedis believe that Prophet Muhammed (pbuh) is not the last Prophet and that Mirza Gulam Ahmed Qadiyani is a prophet. So they are not Muslims.

Salam Yes they are our brothers and sisters if they hold tightly the Qur'an and hadith.But if they join the misguide Islamic branches such as Shias and Ahmadians,they are not our brothers and sisters or they become terrorists

Boko Haram and ISIS consider each other brothers, but they hate Shias, Ahmadis and Sunnis who oppose them

Those words that you have mentioned in your question never appeared in Al-Quran but only Sunnah and Shiah being appeared in Al Quran.

These are , Sunnahtullah and Abraham(A) was the Shiah of Noah(A)

Isis, Boco Haram, Al Qaeda and those types of political terrorist groups are **NOT** Muslim.

Stop listening to Fox News and Rush Limbaugh.

Most don't, unfortunately. that's why there's such prejudice.

Not necessarily, there is a lot of infighting