Very difficult with one’s own family members, and letting sleeping dogs lie is sometimes the best refuge - I doubt they are going to be receptive to your going against the meme. They have been told the opposite, and you may open yourself up to attacks while at the same time doing no real good, except appearing (to their thinking) to be a bigot. in any case, It is not so much individuals such as your niece and Sister-in-law but the greater agenda - imposing itself politically and culturally - which needs to be countered.
Thanks, agree!