Both Sunnis and Shiites are Muslims.
No one has the right to expel anyone out of Islam and to consider him/her unbeliever (or Kafir) so far:
- he/she witness that no god Except one God (Allah) with no partner and that Muhammad is his messenger and prophet, and that
- he/she doesn't deny intentionally any of the Islam five pillars.
As for Sunnis and Shiites:
- They, both, believe that No God except one and only one God, the Creator, with no partner, no father, no son, no companion, no associate, and no resemblance and that prophet is His messenger and prophet.
- They both believe in God Angels, all God prophets, all God holy book, the Day of Judgment, and destiny.
- They both believe in the five Pillars of Islam. They both face the direction of Kaaba when praying.
- They pray in any mosque regardless the praying leader is Sunni or Shiite.
- They may differ in some minor details of rituals but these details are not critical to neither the Shiites nor the Sunnis.
The major difference is historical. The Shiites did not agree on the elected Caliphs (who followed Prophet Muhammad after his death). They believed that the Caliphs should only be within Prophet Muhammad descendants and family. Accordingly, they were supporting Ali Ibn about Taleb (Prophet cousin and the husband of his daughter Fatima) to be the first Caliph. However, they didn't protest against the elected Caliphs (Abou Bakr, Omar, and Ottman) and even Ali, himself, was good supporter and adviser to the three Caliphs until he was elected as the fourth Caliph.
Sunnis constitute over 85 % of all world Muslims. Shiites are majority mainly in Iran and Iraq and some parts of Pakistan.
They both believe the main principles of Islam, like unity of God. However, they have some minor different view points both practically and theoretically, like how to take a Vozoo.
The main discrepancy appeared between Muslim society after Mohammad (peace upon him). Shiites believe that it was Ali (peace upon him)'s right to be the leader of Muslims after the death of prophet of Islam, Mohammad (peace upon him), as he was elected by him. However, Sunnis do not agree with this and say Aboubakr was lawfully elected and it was his right to lead Muslim society on that period of time.
I don't think so because I am Sunni and well people say that u shouldn't because they worship Allah and Hazrat Ali (god forbid).So u shouldn't marry her, unless she becmes Sunni too.