What a dilemma. I faced just this situation a few years ago when a family came to me with three children. I wanted to teach the older daughter and the younger daughter, but didn't want to teach the boy in the middle. There were many reasons why I wanted to teach the oldest and the youngest: they were both interesting children, completely ready for lessons and I immediately liked them. They'd had some "lessons" with another teacher, but I could tell right away that they were going to be successful piano students.
The boy, however, was a completely different story. He was definitely hovering at the edge of the autism spectrum and had difficulty controlling himself. That wasn't what mattered in this decision. He had physical challenges that made piano playing difficult for me. But even more important was the fact that he was fiercely competitive with both of his siblings. He compared himself to them constantly. Trying to teach the three of them would have been a disaster.
Years ago I might have thought the only options were to take all of them or none of them. I've gotten wiser in my dotage. Here was my solution:
I took the mother out for coffee. Sometimes it's easier to have difficult conversations in person. You can make eye contact and use non-verbal cues to convey your sincerity and warmth. We met in a Starbucks to discuss the situation. I explained why I thought that having all three kids playing the same instrument was, in this case, a recipe for disaster. I explained thoroughly why I thought that playing the piano, specifically, would be more challenging for him than for her daughters.
Her son needed to play a different instrument - perhaps the guitar? (The guitar is easier to play and almost impossible to compare to the piano.) She also knew that he was too competitive to be able to tolerate the fact that both his sisters would make progress far quicker and more easily than he would. He needed something that, by definition, would make him special and make comparisons more difficult. I was completely honest with her about my assessment of the situation. And though I was kind and understanding, having raised a difficult boy myself, I didn't take on her son as a student. I took the girls and they did quite well.
Here's the thing: if you can see heading into a situation that it won't be successful, don't go down that road. Any parent would always prefer your honest opinion as long as you are kind, gracious, and offer another solution that will work better. You will be doing no one a favor if you start to teach a child just because the siblings are studying with you. Follow your heart and speak the truth kindly.