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Influence is the power or capacity to have an effect on someone or something.
when a sibling is jealous of another sibling.
Usually when the older sibling is 10, 11, or 12, depending on the parent and how the older sibling and the younger sibling are like.
There is no requirement for a sibling to be in a will.
Sure, here is an example sentence using "sibling": "I have one sibling, a brother named Michael."