It's very challenging isn't it. When one of my sisters was in her late 30s I discovered that she'd never had a pap smear. I was astounded.
We live in a society that can save us from many ills, but huge swathes of people won't immunise, or get the regular scans and tests that are available. I doubt that nagging will do anything. Maybe just once a year you can ask her if she's had her mammogram yet and leave it at that.
Getting a mammogram almost certainly saved my life. My tumour was so deep I would have been a goner by the time it would have made itself known.
I watched my sister die of breast cancer. Let me just say it wasn't like it's portrayed in the movies... I hope your sister reconsiders. K xox