An insult meaning that your sister is actually a transgender man. This insult is so rare it has only been said by 4 people and was buried in the Egyptian pyramids to never see the light of day.
Guy: ur mum gay
You take out a golden box
Guy: Holy fuck hold on man I was just joking please don’t do this
You: Ur sister is a MISTER World explodes
Same relation as a stepsister, but with positive connotations as opposed to the negativity that is typically associated with a Stepfamily. Bonusfamily members are chosen freely and they enrich your life.
My mother remarried after I moved out of the house. I am a good friend with her new husband. As an added bonus, I was introduced to an awesome new awesome friend: my Bonus Sister.