I assume that she reached age 70½ in some year prior to 2016. This means that she was required to take an RMD for 2016. Because she died in 2016 prior to taking the RMD, that RMD becomes the responsibility of the beneficiaries. The requirement to take that RMD does not disappear.
The year-of-death RMD can be taken by the beneficiaries in any combination. Sometimes one beneficiary wants to make a large enough distribution that that beneficiary will satisfy the entire year-of-death RMD. If all beneficiaries want to stretch their inherited IRAs as long as possible, it's probably most equitable to divided the year-of-death RMD requirement between the beneficiaries in proportion to their relative shares of the original IRA.
It sounds like the year-of-death RMD is still outstanding and remains to be satisfied. This would means that you and probably your sister also need to take distributions of the late 2016 RMD and file 2016 Forms 5329 to request waivers of the penalties on that RMD. Keep in mind that all of the distributions made in 2018 are taxable on your 2018 tax return(s) although the requests for waiver must be made for prior years on prior year's Forms 5329
