Hello XXXXXXXX. Thank you for coming to Just Answer and allowing us to help you with your question. The IRS considers a household to be a group of people living together and the head of the household is the person who has can claim a dependent (or dependents) in the group and pays over half of the expenses for the total household. Only one person in a household can file as head of the household. Both you and Sheila have dependents and whichever of you makes the most money can file as head of household. If both you and Sheila make over $3500 a year, then one of you can file as head of household and claim at least Naomi as a dependent. If you make more than Sheila and she makes less than $3500 per year, you can claim Sheila and maybe Sheila's son as well as Naomi as dependents. If Sheila makes over $3500, then whichever one of you makes the most money can file as head of household and claim Naomi (and maybe Sheila's son). The one of you that makes the lesser amount of money will file as single and maybe able to claim Sheila's son or Sheila herself or you if you make less than $3500. Either of you will be able to claim Naomi. The only consideration involving Naomi, is who will benefit the most from claiming her.
Whether Sheila (or you) can claim Sheila's son depends on whether her divorce decree specifies which years she or her ex-husband can claim the boy, that the boy spends over six months each year living with either Sheila or her ex-husband, and that the boy is under age 19 (or under 25 and a full-time student) and meets the income requirements to be a dependent. If the divorce decree doesn't specify which parent is allowed to claim the boy, then the parent that he spends the most time with during the year is allowed to claim him. You can only claim him as a dependent if Sheila makes less than $3500 per year and her ex-husband hasn't lived with the boy at all during the year and the divorce decree doesn't allow the ex-husband to claim the boy.
I hope this answers your questions. Unfortunately, filing status and dependents are two of the most complex areas of tax preparation. Please let me know if you need any more information.
Tax Preparer
4 years professional tax preparation and over 300 hours of training. B.S in Chemistry.