you should encourage your sister to try a different class-taught by someone else- some are like that- but i don't believe it's the majority- i think most teachers are more (accepting) of beginners....especially if it is a beginner class. when we had the sewing school we did not really stress perfection at all- i have one friend who makes great quilts- and her seams are always----horrible- they range from 1/4" up to an inch---seriously! but she makes it all work and the quilts turn out beautiful. i do wish she would start (she's been quilting 5 or 6 years now) to work on technique- but she really has no interest in worrying about it- she does on occassion take a class at a lqs---she just ignores them if they get===picky===she did get up and walk out of one class once---but it didn't slow her down-she just decided if that was the teacher she wouldn't take that class- she would talk one of (us-her friends) into taking it to learn the technique- then we can show her...
tell her not to let one bad teacher spoil the fun==