First off only Allah can decide who is truly a Muslim..
this is one of my issues with contemporary Islam is that the term Kafir gets tossed around freely, when the Qur'an states that this is a grave accusation.
Shia's are our brothers despite what some Muslims may claim.
Alotta Sunni's like to generalize all Shia by saying things like they are disbelievers, they commit shirk, etc etc.
But Shia Islam is just as divided as any other branch or religion. Sure some Shia commit actions that go against the grain of Islam, however most of them pray 5* believe in Allah and the Last Day etc etc.
most of our differences are political or from over exaggerations of ibadah (worship).
I try not to get into this person is a Muslim or that person isn't a Muslim, it's what the enemy wants
I heard a Salafi tell me that Sufi's are not Muslims. But if I were judging by character and actions I would tell you that the one who was more "Muslim" was the Sufi one.