Are you sure that the original account was a Roth IRA and not a Roth 401(k) or similar designated Roth account in an qualified retirement plan? If this was a designated Roth account in a qualified retirement plan, your mother passed after April 1 of the year following the year she reached age 70½ and was not working for this company at the time of her passing, she would have been required to take an RMD for 2018 and that RMD would have been required to have been satisfied before rolling over the remainder of the designated Roth account to an inherited Roth IRA for your benefit. In that case, however, you would have received a code 4H 2018 Form 1099-R reporting this rollover. (Or this could have been a rollover from a traditional account in a qualified retirement plan to a Roth IRA, in which case there would be a code 4G 2018 Form 1099-R.)
If the account she had was indeed her own Roth IRA, there was no RMD requirement for 2018.
