After leaving the hospital, Misumi Mikoto didn't return to UDI, since UDI's headquarters were in Musashi City, and the Eastern University Affiliated Hospital was inside Tokyo Metropolis. By the time she would have made it back, UDI would have already closed for the day.
So, after leaving the hospital, Misumi Mikoto chose to go straight home instead.
Misumi Mikoto, of course, had her own family, but she didn't live with them, opting to rent an apartment by herself.
It wasn't that Misumi Mikoto wasn't close to her family, but rather that her mother wasn't her birth mother—she was her aunt, and her biological mother had taken the entire family with her in a charcoal-burning suicide when she was little. Misumi Mikoto was the only one who survived, and she was adopted by her aunt's family.
