It’s not just the plot and characters; there are many, many shared plot points and chunks of similar text. So yes, ripped off. Here’s a website listing some (not even all) of the correspondences:

http://canadianicon.org/table-of-contents/mirror-images-anne-of-green-gables-and-rebecca-of-sunnybrook-farm/

I agree, Montgomery’s writing is better (but that’s not a very high bar)

was her best Anne of Green Gables? because that ripped off Rebecca of Sunnybrook Farm by Kate Douglas Wiggin