I think Vasus idea of tattoo on pubic region is grear, coz anyways your mom wears saree quite low waist and pubic region is visible many times. So she can get it like a slightly bigger tattoo on pubic region and till waist so that normally tattoo is hidden and when your mom wears jeans and crop top exposing her navel, half of the tattoo will be visible just slightly above her jeans and since everyone will see that only half tattoo is visible, they will know rest half is on pubic region and left gor imagination for everyone. That will be quite Sexy !! And when your mom wears a low waist saree, she anyways pulls it slightly more low waist and displays pubic region and then her wholr tattoo will be visible.