I searched the internet high and low because I wanted to help and also because I wanted to know for myself and the best info I could find
seemed to be at the end of this article http://www.woodturner-russ.com/fsoriginal3a.html which, for the most part, says for DRINKING
Vessels, BARE WOOD is the best alternative but, then you must be sure the type of wood is not highly toxic.
Earlier in the article it also states that Pure Citrus Oils like Hazel Nut Oil are safe but, I would have to wonder if there would be an
oily film on top of your brother's tea.
Good luck and God Bless! Spirithorse