Those of you who were there probably have more of the facts, but I would guess that TPTB came up with the Season 3 war idea toward the end of their Season 2 writing period, as an attempt to buoy their ratings-challenged show. Probably they had no time to weave elements into the previous scripts. C'mon, in "The Expanse," Archer asks Trip (his friend of 10 years) if Lizzie is younger or older than he is.I'm thinking they wrote it quickly.
You are absolutely right: Trip was Archer's best friend, they have known each other for almost a decade: is it believable he didn't even know Trip had a sister? A BELOVED sister? Of course not. They wrote it way TOO quickly. The idea of translating the abstract horror of 7 millions people killed into a concrete tragedy and grief of one well known and well liked character was a good one and Trineer did the best of the material he was given, IMO. The material was not the best, however.
