First, check the zoning. How many homes can be built there? The land may provide a whole new neighborhood for your town. When negotiating a sale with a developer, provide for a life estate in one of the new homes for the handicapped sibling.
The value can be based on an “as is” sale, that is, the developer closes on the property and takes all the risk regarding approvals and infrastructure requirements. That brings the lowest price, but more quickly than a sale “subject to approvals”. In a “subject to” sale the buyer applies for approvals and closes when they are granted. But that is time-consuming and if approvals are denied he may cancel the contract.
Another alternative is for the family members to invest the money and time to obtain the approvals and sell the approved parcels as ready to go. This will bring the best sale price but will require spending fund to have the work done.
All of this is subject to the state of the housing market in your locality, of course. Easy in a strong market, tough in a slow one.
Thank you! That is a lot of good info.