I sit on the regional equestrian association and my daughter is a certified coach so I have strong opinions on this, but I will just go over the basics. In Canada, coaches can be certified through a rigorous evaluation and only after completing rider level 6 (English - Western only goes to 4) which means they can do a basic dressage test successfully and can jump 2'6" correctly. The coaching evaluation itself is a whole day and includes a written test which is quite challenging (they have a manuel to learn and have to get at least 70%), prepared lessons handed in ahead of time and marked, then they are evaluated on coaching skills (they actually coach real riders in front of the evaluators), on their horsemanship skills (they have to lunge w/t/c using correct technique), and on their stable management (including bandaging, for example). Many fail this evaluation, but those who do get certified are truly knowledgeable and well-rounded, and understand the first steps of learning to ride and how they are building blocks for what will come later.
Certified Equestrian Canada coaches are
- fully insured for liability which means they are covered if themselves or their student is injured
- have taken concussion protocol training
- have taken a two-day extended First Aid course
- have taken Safe Sport training
- have taken Making Ethical Decisions training
- have undergone a criminal background check.
Now, clearly, you do not need a criminal background check to teach your cousin. However, liability may be an issue. If she is hurt, do you know how to respond? Can you identify signs of a concussion? You may assume that because this is your cousin, they wouldn't sue you if your coaching led to them being injured, but it may not be their decision. Once an insurance claim is made, then insurance companies get involved and try to pin the blame.
All that said, many people choose to coach without insurance or certification in Canada (and I understand you don't have a similar program in the US). They may be really good coaches too. But I have been riding my whole life (I'm 51 and got my first pony when I was 5) and when I watch my daughter coach, I see all that training displayed. I know how to ride, but I don't know how to teach riding. I don't know what exercises build what skills, or how to make it fun (you'll quickly find that just teaching w/t gets boring real fast for both of you). My daughter is able to break down the components of learning to ride into parts she can teach individually, and she can look at a rider and evaluate what they need to improve. She has learned how to talk about this in a way that will motivate the rider, not deflate them, and measure progress from week to week so they know they are actually learning something. There is no way she could do that without having undergone a lot of training to become a coach even though she has been riding for 11 years (she's only 17 btw).
If you really want to start coaching, can you apprentice under a more advanced coach? You might see that there is a lot more to it than first thought and actually learn a lot from the experience!