Read the verse again. It's not parallel to your example. You weren't firstborn among those many Thai brethren, because then you would have been part of their family.
You want to emphasize firstborn, then go for it.
I’ll emphasize among, as it STILL applies.
Strong's Concordance for firstborn
Word: prwtotokoj
Pronounce: pro-tot-ok'-os
Strongs Number: G4416
Orig: from 4413 and the alternate of 5088; first-born (usually as noun, literally or figuratively):--firstbegotten(-born). G4413
Use: TDNT-6:871,965 Adjective
Heb Strong: H1060 H6363 H7218 H7223
Strong's Concordance for among
Word: en
Pronounce: en
Strongs Number: G1722
Orig: a primary preposition denoting (fixed) position (in place, time or state), and (by implication) instrumentality (medially or constructively), i.e. a relation of rest (intermediate between 1519 and 1537); "in," at, (up-)on, by, etc.:--about, after, against, + almost, X altogether, among, X as, at, before, between, (here-)by (+ all means), for (... sake of), + give self wholly to, (here-)in(-to, -wardly), X mightily, (because) of, (up-)on, (open-)ly, X outwardly, one, X quickly, X shortly, (speedi-)ly, X that, X there(-in, -on), through(-out), (un-)to(-ward), under, when, where(-with), while, with(-in). Often used in compounds, with substantially the same import; rarely with verbs of motion, and then not to indicate direction, except (elliptically) by a separate (and different) preposition. G1519
Use: TDNT-2:537,233 Preposition
Heb Strong: H3789