Sharing a story like that is highly improper. It not only goes against moral norms but may also cause discomfort and harm to the people involved. In a civilized society, we should focus on positive, healthy and respectful topics.
Educate ourselves about proper social behavior and ethics. By having a clear understanding of what is acceptable, we can easily filter out inappropriate topics. Also, surround ourselves with positive and respectful people who won't engage in such discussions.
If your brother watched you draw, it depends on how you feel about it. If you don't mind, you can just continue and maybe even ask for his opinion. For example, you could say, 'Hey, what do you think of this so far?' But if it makes you uncomfortable, you can gently tell him that you like to draw in private and ask him to give you some space.
It's difficult to know exactly. It could be your mutual followers, people who follow similar accounts, or even someone who was just scrolling through and happened to stop on yours.
If your brother watched you study and it's distracting, you can find a quiet place away from him, like your bedroom or a study nook in the house. Just tell him politely that you need some quiet time to concentrate. For example, say 'I really need to focus on my studies right now, so I'm going to study in my room alone.'
Maybe it was some random people who happened to come across your story and found it interesting. Or it could be your close contacts who keep up with your activities regularly.