Definition of immediate family member as per
http://www.cic.gc.ca/english/information/applications/guides/5351E3.aspAn “immediate family member” is considered to be a son, daughter or adopted child.I am sure they do not change definitions between forms so your sister and cousin are not classified as immediate family.
I do not know if they consider you to be working or not. If they say you are not, they may feel you don't have enough ties that you will return. Can you get a letter from the university where you work saying that they are expecting your return when your studies in Canada are finished? That would help.
Your offer letter is good for an acceptance letter. Normally if you would apply for a position and get accepted, that would be the letter.