To a Basenji, cause and effect are immediate. The dog was rewarded for sitting, not for biting. In the same vein, if a dog has been naughty - and this applies to all canines, not just Basenjis - if you call it and it comes back to you, NEVER pumish it. It will associate the act of coming back to master or mistress with the punishment, not the naughtiness.
Your sister told the dog to sit. It sat. It was rightly rewarded.
That it bit you is another issue entirely and as one who has been bitten seriously fewer than 5 times in almost 40 years of life with Basenjis, I can tell you that in every case it was my fault or carelessness.
So I have to ask, what did you do to the dog to get such a painful reaction ? Basenjis can bite when provoked - it is almost always an instinctive reaction. Did you grab it to get it off the table ? Grab it from behind, startle it ???
Don't always immediately blame the dog. If it is not given to biting, something caused it to do so this time.
If it IS given to biting, and I don't get that impression, then your sister has to deal with it, as it is her Basenji.