It appears you currently support yourself and your 3 children and you are agreeing to sponsor your sister and her husband. Seems to me like the answer should be 6 but it is a bit hard to tell from the wy you have stated your facts.
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you plus your children
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Presuming it's you, your three children, sister and husband, 6 is the total number. This means that you make at least $39,487.50 to qualify as a sponsor. Include your 2012 tax return with your w-2 showing you made more than this amount with the form. If you didn't make this amount, there needs to be a different sponsor.