Changing contact email for a single listing

Simple answer is you cannot. In your settings the email is attached to 'your account', not individual auctions. It's also a security thing so others cannot misuse your account.

Yep, As L says you can't for an individual item. There are a couple of messy options if you really must.

Option 1.  Change your account to your sisters email address temporarily. This could be possible if you only have that one listing at the moment. But it will mean all and any TradeMe generated emails will instead go to her. You would probably need access to her email account as well to do it. Because you will need to verify the change from within her email account.

Option 2. Put your sisters email in the description and tell people to use that instead. Not really recommended though as it will open her up for many international scam attempts.

Option 3. Don't publish any email address. Make contact by phone only. (To her)

An easy fix would be to set up a new rule in your own email system to forward the emails concerning the classified to your sisters email account.

Not sure what email you have or what client or system you use, but a google of Forwarding rules for your email will give you some help. If you had a rule for example that matched, If an email arrives that has Classified Enquiry in the message subject line, then forward to email address and you could also add an extra step in the same rule to move the email to another folder so it doesn't clutter your own inbox. The classified enquiry email has the senders email address in it, so your sister can reply directly to the asker.