I am sorry for your loss.
You should contact the Payer. However, since this is from your sister's place of work, it is most likely some form of compensation which would be taxable to you (even though you "inherited" it). This is commonly termed IRD (income in respect of a decedent).
See https://www.irs.gov/instructions/i1041#en_US_2021_publink1000285932
See also https://www.irs.gov/publications/p559#en_US_2021_publink100099579
