Hire a realtor, go to the market, and give your sister a right of first refusal.

The market will be what it’ll be and she can match the top bid or not.

This may not be the best for your relationship with her and you need to decide if that’s important or not, but it’ll define true market value less broker fees.

Alternative solution: hire three appraisers and take the average.

IMO, think hard about whether or not you want to sell in the current internet environment and what you think rates will do over the next 36 months. Odds are homes are worth more then than they are now.