My brother is taking g5 AB horn exam.

I've checked through the syllabue but I just want to reassure myself: are there any restrictions on accompanists?

You mean like age or something?
No.

Oops. I should've made myself clear, sorry.

I meant whether there are any restrictions regarding family members being accompanists. I remember page turners couldn't be family members.

Anyway, I dug through the forum and found many people talking about accompanying their children, or having their children accompany them, so I guess it isn't a problem.

I accompanied my brother for his grade 6 (and actually pulled him out of a few dodgy bits...technically shouldn't really be doing that but there you go, the examiner didn't note it down per se on the mark sheet) when I was only 16, so no problems what so ever (as far as I know anyway).

Do they give any reason why page turners can't be family members?

Just checked the regulations and they can.
Interpreters can't be either music teachers or family members though.