If you bought the house, there is nothing to report now. But, to get ready for when you eventually sell, you need to document your cost basis. The cost basis in your case is 1/3 of the fair market value on the day the previous owner died, plus the amounts you paid to your siblings for their shares. You should probably pay for a legit real estate appraisal to document the FMV, it will cost you money now but save you money in the long term.
If you are renting the house out, you still don't report the purchase of your siblings' shares, but you report rental income and expenses on schedule E. We can help you with that if you need.
If you bought the house AND sold it in 2021, you may need to report the sale as a capital gain on Schedule D, even though you don't report the purchase. Before you do that, we have to ask,
a. did you get a 1099-S at the closing?
b. did you sell for more or less than your cost basis?
c. did you make any improvements before you sold the house?
