I am sorry that happened with your sister, but I am not that surprised. Is your sister riding a stallion? If she isn’t, or has never had to school a stallion she has absolutely no “authority” for criticizing your riding, none at all. I have trained a stallion from a yearling on, and broke him to saddle myself (I had it easy, an Arabian stallion), so please hear me, what you have done is quite IMPRESSIVE.
Family dynamics can be awful, I broke off all relations with my brother, many reasons, but one of them was his constant disrespect, snide comments, and pooh-poohing everything I said, even when I knew a lot more than he did about a subject. Since then my life has been much more pleasant. I know you may not have the freedom to cut her out of your life right now, but in several years you will be a full adult.
The only living things that have any validity criticizing you riding your stallion are: your horse, and your riding teacher. Ignore everyone else. Your horse is showing you where you need to work on your riding, and when your horse is happy with you as he obeys you he is giving you the best feedback, MUCH better feedback than envious relatives and “friends” who never even dared to be as brave as you are. The fact that you can control your stallion when riding with others, especially with mares, shows that you have learned how to control a challenging horse.
When I was younger a lot of my contemporaries at the stable looked down upon me, however at the same time they ENVIED me my horse, who was so pleasant and obedient. When I got my first horse (Anglo-Arab) he had just been gelded (at 5 years old) and only had three weeks of training under saddle, and my riding was at an elementary level (just trail riding), but I did all his training from then on. My horse obviously approved of me, most of the time my riding did not irritate him too much, and we improved together. I never could jump well (my Multiple Sclerosis was active back then, I just did not know it), but I managed to teach him to jump up to 106cm, mostly on my own just following the books on Forward Seat riding by Vladimir Littauer though I had occasional lessons. It took me a while, but I did it mostly on my own, just like I trained three young horses to saddle without any help from an experienced horse person.
I am glad you found a counselor who seems like he can help you when you get another job. I also was never allowed to be good, my mother could ALWAYS find a way to tear me down, belittle me, tell me that essentially I was worthless, and there was always my brother who took his cue from her about how to treat me.
I survived, it was hard, but at age 68, crippled and all, my riding teachers consider me a good rider within the limits of my MS and will put me on horses who need to be reminded about how to obey a person. Calm persistence, not violence, is the way to go! And while my riding teachers do not always agree with me they do LISTEN to me and often add what I’ve shown them to teach their other students. I earned that by learning from my horses, studying the equitation books, trying this and that and keeping only what worked for the horses I ride, and by ignoring everyone who basically demanded that I abuse my horses.
I have ridden seriously for 50 years, and it has only been the last decade that I FINALLY got acknowledged as a decent rider. My horses looked really easy to ride, so everyone thought I could not ride, until they saw me on a really challenging horse.
With the COVID-19 pandemic a lot of the world is freaking out, with reason. Right now is probably not the best time to get any positive feedback on anything important to you, people are lashing out because they are in a state of fear.
It DOES get better as you get older and learn to ignore the people who get the biggest joy of their lives by trying to destroy your joy in your accomplishments. Dear lady, stallions ARE NOT EASY (a few exceptions, of course), and I, for one, AM impressed with what you have accomplished. No one ever is a perfect rider, all we lowly humans can do is the best we can while we try to learn how to get better.
Take care.