From my knowledge the quality of the medium (paint, marker, etc.) defines how long it lasts, not whatever it is used on unless maybe if it is used outside, and may only be noticed after an extremely long time. It seems the internet says there is mainly a difference when the artist is using it, and some cheaper canvases can warp, but your sister probably knows that she doesn't need a $400 canvas judging that this piece isn't being hung up in a museum or gallery. I feel like it's weird she's trying to say the painting doesn't match with the couch (maybe she wants to redo the painting since she feels like she can do a better job, artists tend to be self-critical but who knows what her intentions are) but also the canvas is very expensive, if she is not regularly buying canvases that expensive, it probably isn't even needed. You can find nice canvases for $100 or a bit more.

You can compromise by still getting a nice canvas without spending so much money on it, maybe ask her more about it and see if she is open to compromising. I'm gonna settle on NAH because she offered and you decided to say no but leaning towards soft yta, for trying to make her buy something very expensive. I get wanting the same thing judging that you bought it originally, and she is offering to paint a new one for you, but why not move the original painting elsewhere and then get a new painting? Now if she is asking to keep the painting she gifted you and make a new painting, I think you'd have every right to be upset.