My mother used to tell me, "Least said, soonest mended", so I agree wholeheartedly with AlwaysBusy - just change your seating arrangements and the troublesome sister will get the message.
If she asks any questions, your wife can tell her she feels better sitting on the aisle seat - to stretch her legs/get to the toilet, etc.
Your wife has to distance herself literally & figuratively from this woman if she is causing anxiety.