I'd go for around the 26th - 28th December. Its usually 38 weeks from the conception date.
if you're talking about your sister, she needs to go to a doctor NOW. if she's this close to delivering and hasn't been to a doctor, there could be a whole host of problems the doctor needs to address. if she had already been to a doctor she should have a due date by now, so I'm guessing she hasn't been yet and that is BAD. it doesn't mean that anything is wrong with the baby necessarily, but you have GOT to get pre-natal care it is 100% vital to the health of the baby. Please get her in the car and take her to the doctor. They will confirm the due date.
have you not been to the dr yet??????
If your cycles were 28 days and you ovulated on CD 14 your due date would be right around 12/25 or 12/26.