It sounds like taxes were never paid on the reinvested dividends. That means they are not part of the basis, so the only basis is the fair market value of the account on the date of the previous owners death.
Your expenses would normally be considered a miscellaneous, itemized, deduction subject to the 2% rule, but that deduction was eliminated for tax year 2018 through 2025 by the 2017 tax reform law. While the expenses could possibly be assigned to the estate, there could be practical and legal difficulties doing that. Additionally, since you and your brothers seem to be the only heirs, if you re-opened the estate and charged the expenses of estate management to the estate, that would simply reduce the amount of net proceeds you received. So I think it comes out as a wash, unless there were other heirs. And if there were other heirs at the time of your relative’s death in 2005, reopening the estate would alert them to the missing account and they could make a claim that might be superior to yours.
The good news is that you can report the value of the account as a long-term capital gain, rather than as ordinary income, and capital gains have a lower tax rate.
