Hello vroomhenderson. We're unable to issue a refund to your sister's account. It's just not possible. All we can do is proceed as per our own policies, and investigate the issue in more detail. We have already offered to issue you a Wallet refund.
If we cannot help you resolve it, we will, of course, issue a refund, either in cash or Wallet funds. But the refund can only be issued to you (the buyer), not your sister (the gift recipient).

Right now, we don't even know what the issue is. All that you told us is that it is game-breaking, and quoted your conversation with Telltale, which starts from their auto-responder (in other words, it does not contain your description of the actual issue).
This is why my colleague asked for some details about the issue. Unfortunately he asked for a DXDiag, when your conversation with Telltale suggests your sister is using a Mac - and I apologize for that oversight.

Let's continue this discussion via your open ticket. The refund will not be a problem, but, even though we see that you already talked to Telltale, we need to understand the issue ourselves first. Ideally, we might be able to find a fix for the problem, and let your sister play the game. :)