He's not Paul's birth-brother since Paul would have stated more about him and we'd find him named elsewhere in Paul's writings if he was. We know Paul had a sister and a nephew, but no mention of a brother is made. Rufus did serve as an ambassador for Christ during the late 50s/early 60s and was even sent to Jerusalem with Luke on an important mission, but that wasn't until many years after Paul had been dead for almost 10 years now. If Paul had lived longer than he did, perhaps Rufus would have become a prominent figure in the early church too, but since we don't read about him again after this trip, it seems likely that he died soon after returning from his mission. In addition, there are some differences in how some historians have tried to connect various people through DNA testing. One website claims that someone who has tested positive for being related to Paul also has tested positive for being related to Rufus. However, other researchers have pointed out problems with this evidence since both men were married and had children. They suggest instead that they believe Rufus to be the son of Paul's mother's first husband who she divorced before marrying Felix.