You could also say:
- Si era así... si se descubría que ellos eran parientes de unos... bueno, creía que no podría soportarlo.
The imperfect indicative sounds more realistic, and is always past, while the imperfect subjunctive sounds more hypothetical and may point to the future.
What I don't like is "creía": I'd do away with it and write just "no podría soportarlo", but the original must have required the introduction of that verb of thinking.
Going back to your original sentence, let's see the two variants with their interpretations:
Si era así, no podría soportarlo. (If it was --in fact-- like that, he wouldn't be able to cope with it).
Si fuera así, no podría soportarlo. (Past meaning: If it happened to be like that, he wouldn't be able to cope with it / Present or future meaning: If it was/were like that, he wouldn't be able to cope with it.)
Edit: Thank you, Alan, for your patience and for providing the English version.
On the basis of the English version:
"Was he imagining things? Could all this have anything to do with the Potters? If it did . . . if it got out that they were related to a pair of — well, he didn’t think he could bear it."
it is clear that "did" refers to the actual possibility of that situation having to do with the Potters, and "Si era así" conveys that idea precisely, as does "si se descubría".
Now, the translator chose to write "si así fuera", and I have to say it also sounds right, only rendering that possibility more remote. However, "si se descubriera" would sound, if not wrong, at least awkward, and that is because of the presence of "creía" and because "could" is more of a backshifted future than an actual conditional of the kind we have in present-time conditional sentences:
- If my neighbors were burglars, I don't think I could bear it / I couldn't bear it.
My point is that the conditional sentence both in English and Spanish expresses an actual possibility in the past rather than a hypothetical situation, it being possible to paraphrase it as: If it was the case that it did and it got out that they were related to a pair of — well, he didn’t think he could bear it.
Similarly, in Spanish we can use the imperfect indicative instead of the imperfect subjunctive to express a possible state of affairs in the past. Under these circumstances, I think only "fuera" could fit in there, while any other verb in the imperfect subjunctive would sound odd, my natural instinct as a native speaker being to use imperfect indicative for any verb other than "ser".
To further prove my point that the conditional sentence in question is set in the real past rather than in a hypothetical present or future where only the subjunctive is allowed, let 's imagine a condition with be in the third person singular:
- If Harry was (NOT were) a wizard, he didn't think he could bear it. (Si Harry era un brujo, no creía que podría/pudiera soportarlo.) -> This is clearly set in a real past, hence the impossibility to use "were".
Instead, if the conditional is set in a hypothetical present or future, "were" can be used in English, and "fuera" is required in Spanish:
- If Harry was/were a wizard, I don't think I could bear it. (Si Harry fuera un brujo, no creo que podría/pudiera soportarlo.)