He might feel a bit embarrassed if he is old enough to understand that diapers are for younger kids. But if he is really young, he may just feel comfortable and safe with his sister taking care of him.
The boy could feel a range of emotions. He might be proud that his sister is taking care of him, like he has his own little caretaker. On the other hand, if he is at an age where he wants to be independent, he could be a bit frustrated that he has to rely on his sister for this task. But overall, the relationship between them and how the sister approaches the task would greatly influence how he feels.
If the boy is used to his sister's care, he could be quite happy. He may enjoy the attention from his sister while she is putting on his diaper and dress. However, if it's the first time, he might be a bit confused or even a little resistant at first, but then get used to it as his sister is gentle with him.