[Silent Bob]

I think everyone has missed the point here. the poster needs to take this complaint up with the airline who took his money and that's Sri Lankan Airlines, unless this was a mistake in the posting, I'm just quoting what was said:

7. Some time ago I discovered that I was charged by Sri-Lankan Airlines(!) for $2,067 mentioned above in addition to $750 already charged from my sister's account.

I think that's maybe why you didn't get a refund, because if Sri Lankan airlines is the company that took your money then you should be able to get it back from them. Because as you stated Your sister did everything she was supposed to have done to secure the ticket, allowing you to travel. If indeed this was done by Singapore Airlines, then defifinitely persue the matter as that's a lot of money out of pocket.