Hoshi and Kita aren't twins; they're a lesbian couple.
When Kita is feeling alone, one of her possible lines is "Where's my girlfriend?" (File "Kita_Needs08d.wav".) I don't recall the two referred anywhere in-game as twins, either; Hoshi's profile calls Kita her "soul mate" (NOT "sister"). Indeed, it is possible for the two to develop a romantic relationship.
Yes, the two have a near-identical appearance, voice, and personality. This must be just a coincidence. note
Earth is still inhabited, but only by the poor.
The characters all have a country of origin,because the can't afford to live in space like the rich. Earth is very slowly being evacuated, as in everybody has to pay for their own ticket.