Can flight instructors teach well their own beloved, spouses, children, siblings or parents?
Yes.
About the same as they can teach them to drive a car.
Not in my experience. There's something about driving a car that turns on the machismo even in wimmenfolk. I wouldn't even attempt to teach a family member to drive. Best left to the professionals
I'm teaching my 15-yr-old to fly and he's perfectly amenable to instruction. I can't even get him to tidy his room or comply with anything else, homework etc but he's a different person in the plane. I think motivation has a lot to do with it (on both sides)
TheOddOne.