Two points to consider:
Be sure your family member differentiates between the need to "self-transfer" with a sliding board or "assistance to transfer" with a hoist. Notify SWA of this need in advance and as soon as you get to the airport.
A small wheelchair that can fit down the aircraft aisle is available at the gate and can be used if you need help getting into your seat. Each gate is also equipped with a Passenger Transfer Kit (PTK), which contains a slide board for "self-transfer" and a transfer sling for "assistance to transfer". The sling is used to lift a customer requiring this type of assistance safely from their wheelchair into the smaller aisle wheelchair and then into the aircraft seat. To make this transfer easier, at least the first one or two rows on most Southwest aircraft are equipped with movable aisle armrests.
Also, consider the length of the flight and the need to use the bathroom. While there is often an in-flight collapsable aisle wheelchair available, you will need to be able to make the transfer in the very small bathroom by yourself or with the help of your own assistant. It is a challenging situation and needs to be considered beforehand. Most fliers with mobility disabilities limit their consumption of fluids beforehand, and use the bathroom immediately before pre-boarding. Disposables and catheters can also come into play if necessary.
Wishing you a comfortable and seamless flight!
