Everyone asks if we're twins or sisters what do I do?

Me and my girlfriend we've been together 3 years. I can see why people would ask simply because we have the same body type short hair (mine is blonde and hers is dark brown) and wear gles and wear mens clothes mind you we are lesbains and we are both butch. We do not have the same color hair or face shape or tone of voice. Moving on everyone asks all the time are you guys twins are you sisters it's getting tiring and sickening! I'm quite frustrated. What do I do or say? When we answer no everyone procedes to ask - are you positive you both arnt related- help what do I do?