In the original stories, Sherlock did have an older brother named Mycroft, even smarter than Sherlock himself...however, Mycroft was too lazy to be a detective. He did have a job with the British government and brought Sherlock a few cases of (inter)national significance. Emphasis on few.
Does his expanded role in this series upstage Sherlock a bit? In other words, does it become "the Holmes brothers" instead of Sherlock? Mycroft seems to direct Sherlock a lot, and many of the most brilliant ideas area his, so it's almost like Sherlock is the "second" on the team of Holmes brothers.
If you introduce too many characters as smart as Sherlock, it diminishes Sherlock.