Don't do it.
I was 17 when I had mine done. I knew how to take care of them while they were healing but despite being careful, one of them still got infected and my earlobe swelled up and started oozing. I also used to catch the sleeper hoops in combs and brushes and catch stud earrings on clothing.
How are you going to prevent such a young child from tugging at them if they are irritating her or if they become infected?
It doesn't matter when your mother got you and your sister's ears pierced or your sister-in-law. This is your child. You are responsible for her wellbeing and no child of that age needs their ears pierced.
"DC is 18 months and for about 9 months now she loves earrings etc."
At 18 months, she'll be fascinated by anything shiny.
She'll still be at the stage where she'll pop anything of interest into her mouth, whether it bits of mummy's jewellery, beads, pebbles, worms, snails etc.
I also think it should be illegal to pierce a young child's ears.