Step Family?

So before I get into this big topic, I am going to tell you about some things. 6 years ago my grandfather "committed" suicide and my grandmother was with one guy to the next after. About a year or two ago, she married this guy and only started to act like she cared about his family and them only, the main time she wanted to spend time with her blood related family was if she needed a third-wheel or they turned her down. So I was quite surprised when she gave me a car for my graduation present, but at the same time I am thinking was it suppose to be theirs? I guess you would call my grandma's husband's grandchildren my step cousins? But they are down right rude to me. They cough on me any and every chance they get too, then they call me a bitch and the second im at my grandmas for a family meal their all suddenly super nice to me. I've come to conclusion some people are just down right rude and you don't have to do a thing to them, but I am not going to put up with this...