The people who told you that you can wait until next year are confusing your brother's RMD requirement with your own beneficiary RMD requirement. As sole beneficiary, you are responsible for completing any portion of your brother's 2017 that your brother had not yet satisfied. The deadline for completing his 2017 RMD is December 31, 2017. If not completed by that date, you can still complete his RMD late and file 2017 Form 5329 to request a waiver of the 50% excess accumulation penalty you would otherwise have to pay, but it's much simpler to just complete the RMD by the end of this year.
In 2018 you must begin taking beneficiary RMDs based on your life expectancy in 2018. If you will be age 70 on your birthday in 2018, your life-expectancy from the the Single Life Expectancy table (Table 1) for use by beneficiaries will be 17 and your RMD will be the 2017 year end-balance divided by 17. You'll reduce the 17 by 1 each subsequent after 2018 and use that and the previous year's year-end balance to determine the RMD for the subsequent year. (Note that if you end up completing your brother's year-of-death RMD late, you'll have a higher 2017 year-end balance in the inherited IRA on which to calculate you 2018 beneficiary RMD, resulting in you being required to take out more in 2018 than you would have to if your brother's RMD is completed by the December 31, 2017 deadline.)
Given how close we are to the deadline, you should probably initiate the distribution now. Custodians can sometimes have a delay in processing the request.
