This is a really interesting question to me because when I went to enroll my son in violin lessons, and I mentioned to the teacher that I played, she just said, "well then why am I teaching him? Why don't you teach him?" Honestly, it didn't even ocurr to me to try to teach the kids myself. I knew I'd be able to adequately help them at home, but not teach them alone.

Just because you play, doesn't mean you can teach. Teachers have ways of explaining things....I mean, some things i just don't KNOW - I just DO. You forget over time how you were taught to do them. Or maybe you weren't taught to do them. Or maybe you weren't taught very well. I learned a lot of things just by observing the other kids (like the concertmistress) in the community orchestra I was in as a youngster. I think that's what you mean by not being able to explain in words.

Having said that, now that I've watched the teachers teach my kids for a few years, I probably could teach someone else's child.

So my best advice to you would be to go along with your sister on her lessons if at all possible (probably not if its during school). Or ask her teacher to tell you what she wants her to work on. You will learn so much - I know you will pick up stuff that will help you to even play better, believe it or not. You might even be able to pick up some students.