Sibling rivalry is a painful fact in all families that have more than one kid. Growing up with a younger brother or sister can often be a royal pain. Jenny is definitely within her rights to demand her necklace back from her sister. While it is impossible to avoid all fighting, agreeing on some ground rules regarding respecting property and space can go a long way to reducing the number and the intensity of fights.
Rules like asking permission before borrowing anything from your sister or brother are very important. Knocking before entering a room is another rule that shows respect to another human being.
If we look at what is going on between Jenny and Nicole we see that there is a lot of "putting down" or demeaning of the other. If we stop a minute to think, we quickly realize that I am not a better, smarter or prettier person if I put down my sister. Not only am I not better, I am probably worse off.
Cognitive psychologists believe that emotions often follow actions. In other words, if I do something nasty, I then end up feeling bad, as opposed to first feeling bad, and than doing something nasty. So by putting down my sister, I have not only made her feel bad, I also end up feeling bad. It's a classic "lose - lose" situation.
Mom doesn't do a lot to make things better here. Often the best thing a parent can do when kids are arguing, and neither one is in imminent physical danger, is to let the fight work itself out. Parents siding with one child or the other merely adds fuel to the fire for the next fight. That famous line "Momma always liked you better," never quite disappears.
What Mom can do, and this I give her credit for trying to do, is limit the places where fights can take place. "No fighting around me," or "Go upstairs if you want to fight," may be helpful for the peace of mind of the other residents of the household.
As for the threat of grounding--if Mom doesn't intend to implement it, she might as well not threaten. Hasn't she learned by now that empty threats are useless?? Instead of threatening she might tell the girls how she is counting on them to behave tonight. By letting them know that she relies on them, she is communicating to them her expectations of them that are in fact positive.
We know that kids often live up to our expectations of them. If we anticipate that they will give us grief and trouble, they often do. If we pass on the message to them that we are counting on them, and know we expect good behavior, they usually live up to these expectations.
By telling the girls how she is counting on them, Mom has effectively changed the whole tenor of the conversation. The girls can end the conversation and start the day feeling good about themselves and looking forward to the evening.