My daughter has started 'helping' her brother with his worksheets. She reads the question, he writes her answer

I was the younger one in this arrangement as a child, so I can tell you it feels wonderful and teaches nothing. That said, she clearly enjoys explaining. You could let her check his work after he has done it himself, instead of before. He still does the thinking, and she still gets to be in charge of something.

we had this with cousins one summer. my son did his own sheets in the morning and the older cousin was allowed to mark them with a red pen in the evening. the red pen was the whole attraction. he got more careful because he didn't want her circling things

Does your son understand the work if you ask him to explain one answer back to you? That would tell you how much to worry.

Different rooms until his sheet is done. Then she can check it if he wants her to. If he says no, that's the end of it.