There are several good answers here. I'll only add that there are specific names practitoners call other practitioners depending on relationship.
Very commonly utilized are the terms:
- Si Hing ( 師兄) Elder kung fu brother
- Si Jie (師姐) Elder kung fu sister
because all schools have longer-term, more advanced students who help teach the newer students.
If one is lucky, one also gains exposure to:
- Si Baak (師伯) Master’s elder kung fu brother/sister
- Si Suk (師叔) Master’s younger kung fu brother/sister
Many kung fu films involve the disciple seeking out their kung fu uncle or aunt for additional teaching. This can be extraordinarily valuable in getting a fuller picture of the art, particularly in regard to a certain style through existing relationships. (As an example, the founder of Bagua taught each of his disciples different things, based on their aptitudes, inclinations and body types. This is not at all uncommon.)
You can find a full list of martial relationship terms here: http://www.moyyee.com/about/kung-fu-terminology/
Regarding "Sifu", when addressing one's own teacher, one uses that term independently, or "Sifu [Name]". When addressing a master from another school, one reverses it: "[Name] Sifu". It's also fairly common for non-students to call any master "sifu" to show respect for their skill and/or reputation. In the same way, one can call a student from a different school "Si Hing" or "Si Jeh" to acknowledge the greater experience of that student.
Although there are uncountable schools (and famous rivalries) in the Chinese system, all practitoners are considered to be part of the jiānghú (江湖) or "gallant fraternity". It's important to understand these terms in the context of wuxia and of the Water Magin, one of the four great classical novels, in particular. (In the Water Margin, aka "Outlaws of the Marsh", the heroes are forced into outlawry, but are nevertheless virtuous. This idea was strongly reinforced by Li Jinglin ("Miracle Sword Lee") in the preface to The Major Methods of Wudang Sword, and is heavily emphasized in contemporary mythologization of martial heroes, most notably Wong Fei-Hung.
Essentially, all practitoners are part of one big family, and the familial nature of the terms referenced here is a reflection of that.