I think that getting the opinion of a licensed appraiser is the best way to go.
While I don't want to discount the value of having a real estate agent evaluate the property, the safest/easiest means of getting the property's value is via an appraiser familiar with South Boston properties. Buying or selling a home within the same family can be emotionally trying so any steps you can take to make the pricing opinion as objective as possible is best.
Please feel free to contact me if you need any further advice during this process. You can reach me directly at (617) 899-3162 or via my website www.GoHomeWithMichael.com.
And good luck to you and your brother.