It’s going to depend on the ranking system at her school, what her rank is, and how comfortable she is with the basic techniques that she’s teaching. It’s perfectly plausible that she could, ten years of training is more than enough time to become skilled enough to teach, but you’re going to have to answer the question of why she didn’t sign her niece up for classes at the place where she trains in order to teach her in a safe environment (and on mats, soft landings when learning how to fall are important).
But, this is the sort of really specialized question that we’re not equipped to answer here. I’d suggest checking around at the local dojos in your area and see if any of the instructors there would be willing to answer some indepth questions about rank progression and what rank someone could realistically be at in ten years. My guess is that she’d be pretty high up and perfectly qualified, but I can’t be sure and the only person who could really answer this question is someone who does and teaches judo.
I’d also read the first book in the Protector of the Small series because several sequences in there handle some basic training in throws like learning how to fall without hurting yourself. Much like jump kicks, throws can be difficult to grasp in concept because they have a lot of pieces moving together to make it work. It might be helpful to look at that in a written format. I’d also recommend picking up a self-help martial arts book on Judo. Check the names of the authors online to see what their background is and pick one that goes over the lifestyle in the dojo and the ranking system beyond just technique. You’re going to want a basic overview of the style, training in the style, and examples of how to find a proper school so that you know what kind of school this character trained in.
In many different martial arts schools students are asked to give their time to assist up coming students when they reach a certain rank, so it’s likely that your character already has some experience teaching in an assistant capacity. The important thing to remember is that there is a difference between helping someone else teach a class and running one yourself. Even if your character feels like they could do it, they’re going to run into some pretty serious obstacles as they adjust to their student’s needs. When you practice a lot of the information becomes ingrained and eventually you stop really thinking about the parts and pieces that make it work. An instructor has to back track out from their own experiences and really explain the concepts to their student in a way that the student can understand, and that takes practice.
No matter what this character does, there are going to be a few bumps in the road. Teaching is very rewarding but it can also be very frustrating, especially if the character ends up focusing too much on their own experiences and doesn’t spend enough time thinking about their student’s needs.