“Can I teach my 7 year old sister how to play piano by myself?” - Peterson Normil   Yes.

1. Be consistent - practice 6–7 days a week for 10–15 minutes a day. Doing 1 day a week for 1 hour will not work out. 
2. Focus on reading- Get her several books (piano adventures, Alfred, Hal Leonard) and have her constantly focus on building a visual vocabulary of intervals. Reading by interval instead of note names will promote independence and also help her start with the right habits. She shouldn’t read by note names, although knowing the names of notes is important. 
3. Make it a bonding experience - Focus on her personal growth and don’t make it about perfection. Allow her to make tons of mistakes and try to help her develop a system of auto-correcting by learning to recognize what’s right and wrong from her reading practice. Spending all week on getting one piece right may feel right, but the long term effects of always doing that doesn’t work out. 

As a bonus tip - having her watch videos and copy what others do or simply having her play pieces that are too difficult will be the fastest way to get her to quit.   If you want to see a parent who doesn’t know music teaching his child, here is how our methodology works. It’s based around parents teaching music to their kids while teachers teach the parents how to be a step ahead.   Good luck!   The tortoise always wins, Julian