Scott Kelly said Mark is holding up "very well" and tells him that his wife, the wounded congresswoman, is improving every day in rehab in Houston.
"He's doing about as best as anyone could in this situation," Scott Kelly told The Associated Press in an interview from the International Space Station. "He's concentrating on Gabby's care. But he's also been going back to work."
Scott Kelly, the space station's commander, said he expects his brother to make a decision on his NASA flight status "fairly soon." Although he declined to elaborate, his remarks suggested that his brother will choose to fly.
Mark Kelly will need to consult with NASA management, on whatever he decides. He is the commander of Endeavour's final flight. For now, another astronaut is substituting for him during training.
In an overnight Twitter update, Mark Kelly wrote, "Today was a huge day for GG. Lots of progress!"
The Kellys are 46-year-old identical twins; both are Navy captains and joined NASA's astronaut corps in 1996. Mark has flown three times aboard space shuttles; April's mission would be his fourth.
Scott Kelly said their Navy background as high-performance pilots enables them to put their personal lives aside, when necessary, and focus on the job at hand. He said he's managed to do that aboard the space station, as the skipper, ever since the Jan. 8 shooting in Tucson.
"My brother certainly is very good at that," he told the AP. "If he does choose, and NASA management chooses, for him to fly this mission ... I am absolutely 100 percent confident that he will have no problem fulfilling his responsibilities the same way as if this incident would have never occurred."
Scott Kelly described his brother as a logical, thoughtful guy who is weighing all the considerations, and is leaning one way. The two have discussed the options in depth.
Scott Kelly said he's spent lots more time on the space station's single phone - and checking e-mail - than he did for the first three months of his mission.
Before the shooting, he spent his spare time looking out the window and snapping pictures of Earth.
Even with all that's unfolded back on Earth, Scott Kelly doesn't feel compelled to rush home. He will return in a Russian Soyuz capsule in March.
"I recognize that I have a responsibility here and that's kind of the cards I was dealt," he said.