“if I knew then maybe I could cheer sister up because, at the minute she is taking a tantrum, it’s lasted a long time, it’s not nice seeing my little sister like this :’(.”
Wow!
Walk away from temper tantrums or she will escalate the tantrum on purpose until she gets her way.
Do not give in to that. Tell her she can be upset but not to that point of behavior that is disruptive
I think that is little girl at 6 years of age has already learned how to manipulate through throwing tantrums.
What works for her in the past she will repeat until she gets her way.
Are the parents not disciplining her properly?
Not YOUR job to handle adult parent duties.
Tell her to talk to parents and leave you out of it.
Let parents handle by no bribes but honesty and respect for parents and rules of the household. That is the only way she will learn to behave better.
By the way this little girl know exactly what effect she has on YOU and her parents.
If SHE CAN make a situation intolerable enough to get what she wants she will.
Stop showing her THAT YOU are upset..she is using that as leverage.
Walk away and live your own life without this drama.
When left alone she will soon discover that no one likes her this way and hopefully she will change for the better.