@cromat they did explain a little in anime... both of them have lived in same apartement when their mom died. their father so busy that rarely have been seen, it was until taihei graduated from high school. just like kanau and hikari right now
their mom died at young age when her brother is still in elementary while umaru still so young that she cant remember anything about her mother while taihei only remember a few from his past. when taihei graduated he actually offered a job by his father, but he rejected. Umaru still live in that apartement with his father until graduated from middle school then sent off to her brother apartement by her father bcoz her brother can maintain her grade and start in school that same as taihei