Oh boy, my first thoughts are these -
First, you have to figure out the basis to know whether you're dealing with a gain or loss since the sale price is so low.
If a loss then you would report the entire in 2021, the year of the sale.
If a gain, was this family cottage ever rented with depreciation taken? Then you'll have the recapture in 2021 and (possibly) no income again until in 2024, but with it being a sale to related parties you need to know if the brothers have already disposed of it before she got all of her money. If they have already disposed of it, then this client must recognize what they realized as if she received it herself. You can check out the IRS Pub that deals with installment sales for more on this.
Also, if the transaction results in a gain, then you definitely have to use installment sale reporting which is the default method of reporting. The opportunity to elect out of installment reporting is gone because that election has to be done on a timely filed tax return, including extensions, for the year of the sale.
Will this woman ever receive the money from the brother that is technically in default, or will his non-payment ultimately result in the terms of the installment sale being redefined?
With installment reporting, it sounds like you are looking at 6252 and Sch D for 2021 to document the transaction on the return with no payments in that year, and then in 2024. If it was ever rented and depreciated, then you possibly have recapture income for 2021 and add 4797 into the mix. If no installment reporting because it's a loss, then only 2021.