Will your mom be taxed to bring her inheritance funds that she received abroad fifteen years ago and left invested abroad into the US?

If your mom was a US citizen or resident, then any income she earned on the funds while they were invested abroad is taxable and should have been reported on her return in the year the income was earned.

And if she had foreign bank, investment, or other financial accounts, she was required to file Form TD F 90-22.1, Report of Foreign Bank and Financial Accounts every year the accounts were open and had more than a certain value.

If she received more than $100,000 from a foreign estate she was required to file Form 3520 when she received it.

Any year she had certain dealings with a foreign trust, she was required to file Form 3520.

There is no tax per se on bringing foreign assets into the US, but if she failed to pay appropriate taxes on income from foreign sources or failed to meet the reporting requirements on her foreign financial dealings and they catch up with her, there could be severe penalties.