Newsflash: Basis to relative “buying” my home

My sister-in-law is purchasing a home owned by my wife and I. To avoid agent commission we are planning on doing this via deed transfer. We would have a contract for the deed transfer. This is in Texas if that’s relevant to anyone. The SIL would be sending a wire payment directly to the mortgage holder based on a payoff statement I provide her. The main reason we were going to have her do it was to avoid multiple bank transfers and to speed up the process (avoid as much daily interest as possible). My question is, since this is not a typical sale, what is the SIL’s cost basis for capital gains purposes (when she eventually sells)? Is it just a transfer cost basis? We understand that this situation would make it very easy for either party to screw over the other but that’s not my question. Any official sources you can provide would be so helpful! I am an almost-CPA but had a hard time finding answers to this.