Inherited house basis calculation with no appraisal at time of death

House was inherited by 4 siblings in 2016, sold in 2017 after $25k improvements, painting, flooring etc...

Did not get appraisal at time of death. How do we calculate basis, and since the proceeds are split among 4, do we split the basis as well?

Is the buyers concesssion (3% of sale price) subtracted from the sale of the house?