Re: Emotional issues with kids
Even if the kids fight , they love each other to death and the thought of losing a sibling is devastating for those little kiddos. It is natural.
My daughter and My younger son used to fight , she would be the perpetrator most of the time, because of being daddy girl. Once we went to Pizza Hut , my son was waiting to pickup his order and go. We came out and I started the car and started rolling it, so my daughter started crying that why am I leaving my son here. I told her because you fight with him all the time. She was 3 at that time. She said that she will not fight with him anymore. Let us wait for him.
Now she is eight if I try the same trick now it will not work. 
As for her being upset about others taking her sister to their home is a traumatic thought for her. She knows them and knows that they are decent people so they are not lying. 
and they are serious about it.
She will get over it at an older age and would know a joke from a serious comment. Not at this tender age of 6. She is still a baby.