Um, I would go by what doc thinks is best....not just because someone wants it to happen. I understand it's your sister, but don't you want your shower so you have what you need and baby to cook as long as it can??
If it is a health situation your doctor might not take into consideration of your family wanting to see her born and not waiting till there 9-10months because at that point she wants to induce you for a reason . But I'm sure if you explain it to your ob you can work something out a few days different shouldn't hurt . Try not to worry about everything things will work themselves out