Then he will more than likely have some tax to pay. He has "basis" in the house would be his share of the fair market value at the time his mom passed away. He compares his share of the sales price to the basis, subtracts his share of the expenses, and the balance is long term capital gains.
For example, if the house were worth $150,000 when the mom died, and his share was $50,000, and it was sold for $300,000 with $21,000 of closing costs, his share of the sales price would be $100,0000. His share of the closing costs would be $7,000. So, his sales price, $100,000 less the $7,000 of selling expenses, less his cost basis of $50,000 would leave him with a $43,000 gain, which would be long term capital gain, taxed at 15%.
He would be subject to the tax. IF you filed returns as Married filing Separately, you would have no tax obligation for this, and there would be nothing you would owe on this transaction. IF you file as Married filing Jointly, then the IRS has what they call "joint and several liability", which means that even though it is his transaction, you are responsible for the tax just as much as he is, since you filed together.
I hope this answers your question! If you have any more, or have specific numbers, I will be happy to answer.
Thanks! Have a great weekend!