your brother is sorta saying...
"if we sold it on the open market, we'd pay a Realtor 6% to sell it, ending up with $225,000 in hand, if we ended up doing that... and therefore it's reasonable for me to pay that amount to the family".
But the appraisal is supposed to represent the "purchase" price of the property... the market value price... not the "listing" price. Certainly they're no way that you can be assured that if you listed today at $255,000 (255 x -6% = apprx. $240,000)... that you'd GET full list price... or even that you'd get $240,000 in the end. The market is still unpredictable out there.
I'd suggest having a few Realtors come through, and have them give you a CMA (comparative market analysis) to see what they think the property would realistically list / sell for, and then base your "sale" price to your brother, on that.
If the brother isn't willing to pay that... then I'd say, have one of those Realtors list the property... with a "holdout", that if you can't get at least $225,000... your brother has the option of purchasing it at that price.