As a parent have you fallen into the pattern of what you do for one child you do for the other? Do you try to ensure your children are treated equally when it comes to attention, time and things?
Perhaps it's time to look at treating your children fairly instead of equally. Fair and equal are not the same thing. Treating children equally means you treat them exactly the same. Treating children fairly means you take into account the individual needs of each child.
For example, your youngest child needs a new pair of shoes. The sibling says, "I didn't get a new pair of shoes. That's not fair!" It's important for you as the parent to point out that their sibling outgrew their shoes and needed a new pair so that was fair. Getting a new pair of shoes, when they're not needed, simply because a sibling received a new pair would be equal treatment not fair treatment and in our family we believe in being fair.
Using a fair approach instead of an equal approach might be something you are not currently doing or perhaps you are challenged with doing it as consistently as you would like. Take a close look at why you treat your children equally instead of fairly. Ask yourself some questions. Do I not want to hurt my child's feelings? Does it bother me to see my child disappointed? Am I afraid my children will think I love one of them more than the other? What message am I sending to my children when I treat them equally all of the time.
Are your children learning from you that they should expect equal treatment irregardless of the situation? Does that reflect how society treats us? Life isn't always fair and it certainly doesn't always treat us equally. That's an important lesson for parents to teach their children.
I encourage you to take small steps in changing the way you treat your children. Strive to treat them fairly and not equally. If you're unable to make this change on her own it might be time to enlist the help of a parenting coach or a counselor.