OK, let's sum up.
1. If your mother was a US person, and gifted you a house that was worth $150,000 at the time of the gift, she was required to file a gift tax return for the year of the gift. You can do that now; you would write "deceased" and the date across the top and sign in your name with "signing as administrator". However, no gift tax is owed unless her lifetime total of gifts plus estate was more than $5 million. So I don't really know if the IRS would care much. You can run it by the tax advisor you need for the house sale.
2. For the house sale, you owe capital gains tax if the sales price is more than your cost basis; the value on the date of the gift is ignored.
For determining cost basis, it matters how the gift and deed were worded. There is a concept called "life estate" -- the previous owner retains a right to live in the home, can't be displaced, and remains responsible for taxes, maintenance, etc. See this http://www.susanmooney.com/?page_id=530
Because you don't really have any rights with a life estate until the life tenant dies, the IRS considers that you inherit the home. That means you inherit a stepped up cost basis -- your cost basis is the value of the home on the date your mother died. That means that if you sell it shortly after she dies for roughly the same price as the value on the date she died, you owe no capital gains tax at all. You might even have a deductible loss -- suppose the fair value on the date she died was $90,000, and you sold it for $90,000, but paid $5,000 in real estate commissions and transfer taxes. That means you have a capital loss of $5,000 and can deduct that against other income. If you wanted to claim an even greater loss, I would be very careful to document the fair market value on the date of her death, with a qualified appraisal or other documents. Remember if you are audited the IRS won't give you any basis you can't prove.
On the other hand, she might have given you the home "in fee simple" -- a straight gift with no strings attached. In that case, she also gifted you her cost basis -- what she paid to buy the land and build the home (you can't count her own labor she did for free or volunteer/ friend labor, but you count amounts paid to contractors and for supplies. You have a taxable gain if the selling price is more than the cost basis. You will need to do your best to document your cost basis because, as said, if audited, the IRS does not have to give you any basis you can't prove.
You may need to see a tax attorney to determine if your deed or gift letter meets the very strict IRS rules to show that it was a life estate entitling you to a stepped up basis.
*Answers are correct to the best of my ability at the time of posting but do not constitute legal or tax advice.*
