My sister did this. To the extent that she told Social Services, and asked for a special meeting with her siblings (mostly in foster care) to say goodbye and pass on medical information that we urgently needed. She spent hours messaging anyone with a name similar to mine, saying she was dying and desperately needed to talk to me. She messaged my work, my ex partner, my friends, my old school, anyone.
She'd shaved her head, and she "created" a lump just big enough to show under her clothes. She walked slowly with a stick, she looked completely pale and washed out, she kept slurring her words. She told us she'd had breast cancer that had spread, that it was now in her spine. That she had kidney failure too, and doctors had told her her organs would fail. She bought official looking letters that stressed how vital it was that we had ourselves checked. We were "Very likely" to have the same conditions, according to the letter, and if any of us were lucky enough not too, we could potentially save my sister for a few months if we gave her blood and a transplant. She sobbed as she handed them out.
In actual fact, she'd shaved her head the week before, just after requesting the meeting. She didn't eat for two days before meeting us, so that she was weak. The lump was some kind of sporting device for protecting a nipple piercing during sports. She didn't need the stick. She'd stolen and edited the letter online.
Social Services hadn't investigated, so couldn't help. My little sister was only 8, and her adoptive family sobbed. They were inconsolable that she might die. She went through a lot of testing. They wouldn't believe she was healthy. They were petrified she'd be like her sister, and suddenly be terminal one day. I had a lot of tests too - I've got kidney and spine problems and the doctors were concerned.
She is fine. She never admitted that she made it up - the closest we got was her saying she might "have misled us accidentally". She wouldn't apologise for the endless testing she put us through, the stress.
Every year and a half, she's ill again. It's back, she's going to die, we'll all be sorry that we don't believe her. It's exhausting.