Financial talk

i love my boyfriend very much...we've been living with my family [and sadly don't have to means to move out]...the thing is, my younger sister is incredibly obnoxious and thinks the world revolves around her...and my boyfriend is a bit of a know-it-all...for a few months now my sister has been purposefully picking fights with my boyfriend and a couple of nights ago they kind of had it out...my mom insists that it is solely on the fault of my boyfriend because he is older than my sister...she insists that i need to talk to him and tell him that he's being a jerk...it isn't like he is trying to be a jerk, i guess he just comes off that way sometimes...but i know that i do have to talk to him about being a know-it-all...it is kind of driving me crazy...and i don't want to have my mother come to my place of work to trip the crap out on me again...we're trying to save money to move out, but that's a whole other story...i just don't know what to do...