It has just dawned upon me, but... Why is tea the only thing Chinatsu can prepare without making it radioactive? Is it because of her sister?
Chinatsu originally wanted to join the tea ceremony club. The club had disbanded, allowing Kyouko's amusement club to use their old clubroom (illegally). Kyouko managed to convince Chinatsu to stay with the amusement club until she gathers enough members to restore the tea ceremony club. She's been practicing making tea ever since then.
My guess is that she may have been interested in tea ceremony since before joining middle school and may have practiced a lot at home (with or without her sister's help). In other words, her tea doesn't kill because she's got a lot of experience making it, as opposed to her art and cooking skills, which she probably doesn't practice very much if at all.
Just a guess.