Most of the retconning has occurred as the present, real-world timeline has overwritten the future-history timeline of SR.
E.g. as far as I can remember, the 1999 Seretech Decision didn't actually happen. (OR DID IT?!?!?!)
Keep in mind, the timeline of SR has advanced in the Core rulebook and in many of the various sourcebooks that come out - there's usually a metaplot event or two that becomes cannon. (E.g. the Arcology Shutdown)
Supposing that the "started turning into the creatures" part you quoted is from p.50, I'd like to draw your attention the specific mentioning of Goblinization in the introductory part of the book:
Then, in 2021 (...) That’s when Goblinization struck. And it was not
pretty. Where UGE had created interesting-looking newborns,
Goblinization struck people of all ages. The most noticeable symptom
was blinding, mind-numbing agony that came in waves. This lasted
twelve to seventy-two hours while the victims changed shape, grew
tusks and/or sprouted horns, and maybe quadrupled their body mass.
Which is how the orks and trolls came back.
Shadowrun 5th edition core, p. 20
So I think yes, Goblinization is still there as it used to be in earlier editions which you can, in my opinion, consider semi-canon: as long as something is not mentioned in SR5 or is not invalidated by what SR5 says, it's canon, though you'd of course have to keep an eye out for changes presented in later and upcoming publications as well, not just the Core... if you wanted canon.
As for your backstory, let me give you a slightly elusive answer (besides noting that as far as I can remember yes, people can undergo the change later, though that's quite rare these days, and yes, you can have a sibling from a different metatype) : Ask your GM. Seriously. There are always anomalies (people born with two heads and so on), there are rare and unique stories (that make them even more interesting), and you're playing a PC whose story is indeed supposed to be rare and very interesting.
Best Answer
I've found this wikia site rather reliable so far - but, of course, I might be wrong, and other answers may provide more up to date and more precise information.