A couple of thoughts come to mind regarding your situation. First of all, you mentioned that at MDO he gets into mischief if not kept busy. Could he possibly be bored and not challenged enough, especially if he's very smart? He may need a more stimulating environment or curriculum than his MDO program is offering him. How are his interactions with his peers? Does he play well with others or does he seems to struggle a little in that area? His peer interactions could be related to how he interacts with his sister. As for the situation with him and your daughter, he sounds like he may be feeling "powerless", and by creating big "crashes", it probably gives him a feeling of accomplishment or strength. I also wonder if he feels a little jealous of her or feels like he has to compete with her for attention. Negative attention is better than no attention in a child's eyes. He may have some feelings inside that he doesn't understand and doesn't know how to express appropriately in words or actions. It might help to label his feelings to him when you see them, like if he seems mad, say calmly and matter-of-factly, "You seem really mad", or "you seem sad". It may feel silly or even sound silly to say those kinds of statements to him, but in time, kids will sometimes tell you why they feel that way after they understand what it is they're feeling. This may be a little difficult, though, depending on how well he talks at his age.
I feel for your situation and hope these thoughts give you some ideas of things to explore to improve things at home between your son and daughter. Best wishes to you!
