I worked with a relative, can they write me a letter?

I’m applying for grad school to study education and I need three letters of rec. One should be from someone you’ve worked for/with. My best teaching experience is that I’ve volunteered for over a year at a high school. This school is where my sister works and I volunteered mostly in her class to be honest. I didn’t work a lot with the other teachers. She is really the only person I’ve worked for a lot in a teaching environment, and she is the only person I can think of to write one for me. She knows more than anyone about my ability to teach, my drive, the way I work with kids because she was my supervisor. I was working with her in a professional capacity, and the rec would mostly include a description of the things I did while volunteering, but I’ve heard that its not a good thing to use a relative.
Do you think its ok?

Talk with her about this. She may have ideas for good ways to work around the issues.