Cruel, definitely. But even well-meaning family could make the wrong call in a situation like this if they didn't understand the importance of HRT to their trans relative, didn't understand the implications of stopping hormones, or got bad advice from a biased doctor. It's a good idea to make sure your next of kin is not only supportive of your identity but really understands what you would want and what transitioning means to you.
This!
Remember, too, that if you ever are in a coma, that is going to be an extremely stressful time for your loved ones. Well-meaning people under stress don't always make the right decisions, no matter how much they love you, no matter how much they want to honor your wishes. Their big focus is going to be on you making it through alive. Everything else is gravy to them.
So make it easy on people. Talk to your next of kin now, while you're not in a coma. Be explicit about what's important to you. Write it down. Consider having an advanced healthcare directive drawn up. When my friend signed the POA papers we had a very explicit conversation about what was important to him. We talked about his death. We talked about him being in pain. We talked about whether or not I should contact his completely unsupportive parents. I do not rate this conversation as the most fun conversation I've ever had in my life, but it remains one of the most important. He told me things I would not have guessed right -- and we had known each other for a decade at that point. He was very vigilant about telling his doctors that I had POA, too. He was very vigilant about putting my name down on forms as his next of kin. He didn't want there to be any question of who was in charge if he could not be.