Sorry to hear about your sister's unfortunate diagnosis. You need to be very aware that your sister may not be able to travel comfortably two or three months from now, either because of her primary condition or because of the effects of treatment (chemotherapy plus radiation therapy) that she may be receiving.

From the NYC area, you might also try to see what JetBlue can do for you.

The goal to travel is a good one, but you may also consider something that doesn't involve air travel... something like a week on the oceanfront at Virginia Beach (great weather till late September) with side trips to Busch Gardens in Williamsburg, Old Williamsburg, and Jamestown/Yorktown etc.

Or Niagara Falls/Toronto.

Just considerations that allow you to avoid the uncertainties of her health and non-refundable air travel costs.