For starters, the costs/fees associated with selling your home include the amount required to pay off the liens on the real property, such as mortgages, judgments, taxes, as well as closing costs including but not limited to brokerage commissions, realty transfer fee, and attorney's fees. The commission fee is negotiable and percent agreed to is between you and the real estate broker you hire whether it be your friend or another agent.
Factoring in the costs and fees associated with the sale of your home, does not determine its price. What determines its price is recent sold comps of similar type in the same neighborhood. What you owe, what commission you are charged and any closing costs you pay unfortunately has nothing to do with what your house is worth. You need a real estate agent to sit down with you and show you sold comparable properties - that is what determines price, along with location and condition.
There's nothing that can compare to the services a real estate agent can provide you with, which goes way beyond just listing the home, but where did this buyer come from? Were you a for sale by owner? Did the buyer's agent (sister) bring the buyer to your home? Did you agree to pay the sister/agent a commission?
Do you have idea what your house is worth? I recommend contacting your friend who's an agent or other agents in the area and determine what your home is worth before proceeding.