Is this a power tactic?

So, I am telling my brother about someone that has been abusive/disrespectful to me (my sister he is not close to who lives in another state) and he is agreeing with me all through the conversation and then suddenly shifts, does a u-turn and says 'actually no I think you're the *****' (in the past in this kind of scenario I would get all timid, compliant, saying 'what' sweetly girlishly etc. like I had done something wrong but this time I said ertively, strongly and because my confidence was up 'Why did you just do that?') He said 'because it is fun' and starts laughing..To me, it is like taking the rug from under my feet and I don't see it as fun. It destroys a feeling of safety, security, trust. I have social anxiety disorder precipitated by my family of origin. I am trying move out of home at the minute. But, would I be correct in saying that doing this to a person for fun is actually trying to undermine, bully, make someone nervous and uncertain?? I have no friends. I am 21. I am the youngest in my family and they are all disrespectful to me from a young age and I am not sure why I have no humanity in their eyes almost. I am kind to them but when I stick up for myself I am always wrong apparantly. The only reason I had the confidence to be like '**** you' metaphorically on this occasion was because I had a boyfriend and that was at least one supportive, edifying relationship in my life at that time which gave me a boost in confidence and ertiveness. But, as soon as things were getting a little better for me, he kicks me..??????