Griefers always say that a sibling or cousin of theirs logged into their account and started griefing. I hear this excuse alot  but if it really does happen, I have some suggestions for preventing this.

1. Do not click the "Remember Password" box when logging into Minecraft.

2. Lock your computer

3. If you need to leave, do not be afk if you have cousins or siblings near you, Just log out.

4. If your older then them, just scold them

Hope this helps!