1997 the one son gets the step up in basis. The added brothers were gifted their shares from the one brother. That's your baseline.

"What is the brother that died in 2022 correct cost basis for his estate return?"

The gift in 1997. In 2022, if the two remaining brothers inherit the 1/3, they each get a step up of basis for 1/6 from the dead brother's 1997 gift.

"both remaining brothers are splitting the deceased brother share of the home sale equally."

But: Did they already own it? The title conveys ownership. When 3 people own the property, it is JTROS or it is TIC. As Tenants in Common, that 1/3 stands alone and passes on to the estate. As JTROS, the death of one person leaves 2 remaining owners.