Ok, so all Harry Potter fans I NEED EVERYONE to go back and read Order of the phoenix because in the books Sirus' death is so fricken unhinged!!! Bellatrix did not mean to unalive him.. at that moment everyone stopped fighting but these two!!! they were laughing and taunting each other!! It was just cousin rivalry... I honestly think they just went back in time when unaliving each other was a part of the family dynamic and when Sirius accidentally died I can guarantee that Bellatrix was thinking "Did this mf really just die on me?" They were honestly just having fun!!!!