Her comments came after a highly charged parliamentary debate on Wednesday night, in which opposition MPs criticised the premier for using "dangerous" and divisive language as he defended his EU strategy.
His sister particularly criticised Johnson's remark that the best way to honour Jo Cox, an opposition Labour lawmaker murdered by a Nazi sympathiser during the 2016 EU referendum campaign, was to deliver Brexit.
"I think it was particularly tasteless for those who are grieving a mother, MP and friend to say the best way to honour her memory is to deliver the thing she and her family campaigned against - Brexit," Rachel Johnson said.