I'm sorry for your SIL.
Send her here!
It takes more than skill and a desire to pass it on to make a teacher. Even more than technical ability, it takes empathy, a positive attitude and a love for providing encouragement.
Students will eventually get the technical aspects of almost any skill - but beginners need encouragement even more than they need facts and technical details. Derision and tsk-tsk critiques may salve the ego of a teacher who needs that kind of reinforcement, but they do nothing for raising up students and giving them confidence.
If you're going to be a teacher and you want to make a living at it, plan to put the catty remarks and critiques away. Cultivate a positive attitude.
If a student comes away from your class with a less than perfect finished project but she likes it and she has lots of enthusiasm and courage to try it again, you have a convert.
If you have snippy-sniped your superior attitude at them, they may finish a class with perfect 1/4" seams and a very nice quilt that they absolutely hate. They may have also developed an intense dislike of anything quilt-related.