My headcanon: Both of Mako and Bolin's parents were from old-fashioned, well-off families, but neither family was too happy about the two of them being together - she was Fire Nation, he was Earth Kingdom, and they lived in a former colony. And so the lovebirds eloped. Mako was rather unexpected, and too soon for them to be prepared financially for a baby. They were in a stable place when Bolin came along, but then the family had another set back when both of the boys began to show they were benders - neither of their parents were benders, but they were responsible so they set their boys up with teachers. One evening the parents went together to go pick up the boys from their practices. It was cold out and Mako was suffering because he'd once again managed to char his jacket during practice. His father gave him his scarf. They were on the way to get Bolin when the mugger attached. ... And that is my headcanon... :T