Victa,
I'm sure you know, but there's flying and teaching. They're two totally different skills.
One thing you might have working against you is that you're teaching your sister to drive. If it were not a relative then it may be different as you could start with a clean slate and not carry over any baggage from being siblings.
It might also be a clash of personalitles, because you can't teach everyone. They may need a different person to learn from.
That being said, I was able to train my brother and he got a PPL. I was ready for it to be a failure though, but it worked out quite well. That is without a doubt the aviation achievment I am the most proud of.
FWIW, I've not done too much instructing and it was all part time, but I learned a lot from it.
Anyway, keep at it and talk to your sister about it if you want. she might give you insights that'll help you and then you'd be teaching each other.
BTW, your post suggests that you have what it takes to be a good instructor.
Also, keep your eye on your students because sooner or later they'll try and kill you
