Some random thoughts:
Car rentals can be obtained for under $30/day. Parking is another $15/day. Plus you'll be able to eat some meals offsite, another savings.
The Disney Dining Plan (DDP) is up 20% in 2013. Even with Disney's inflated Table Service price, those who use DDP today have a hard time spending that amount. No matter how you do it, eating at WDW is expensive.
WDW has been offering "Free Dining" and "Room Only Discounts" throughout the year, including in October. See here for historical discounts:
http://www.mousesavers.com/historical-information-on-walt-disney-world-resort-discounts/
Even with discounts, staying offsite for $100 will be much cheaper but if you wait for one of these discounts, it will close the gap.
Beyond the money, there are other considerations.
I assume your sister-in-law's timeshare is presumably much bigger than a room at Port Orleans Riverside (POR). If space is important, that's another consideration.
There's the magic of staying inside the WDW bubble; no real world interfering with your vacation.
POR is a lovely resort with a fun pool and good food court.
By the time you go in October 2014, FastPass+ should be fully rolled out at all 4 WDW theme parks. Right now, it's looking like onsite guests will get first access to FastPass+ selections. If you want to have FastPass+ for high-demand/low-capacity attractions such as Toy Story Mania or Soarin', you might have to stay onsite. Certainly it will improve your chances.
I wish I could give you a definitive answer. I think it all comes down to how badly you want to stay onsite.