The only problem with "keeping her fursona original" is that, as I said, it was never original to start with

. The CS rules state that similar markings and colors are not considered theft, unless the combinations of those are highly original and rarely ever seen.
Creating a drawing with some of the same colours as another drawing, whether you have seen it or not, is NOT theft. (This does not apply to direct copies of artwork.)
I don't consider you rude, but if you or your sister are that bothered by the similarities, you should always just report and led the mods decide whether or not it's theft :3. That way, fuss is avoided and the situation is resolved by the staff.