You have to pay more than half your household expenses. So the first step is determining what your "household" is and if your household is separate from your in-law's household.
The second step is adding up your household expenses (rent you pay, utilities and phone if you reimburse them, food you buy). Note that "household" expenses don't include travel, clothing or medical expenses, those are personal, not household. See page 8 here <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-pdf/p501.pdf">https://www.irs.gov/pub/irs-pdf/p501.pdf</a>
Then the third step is to consider whether you pay more than half of your household expenses, taking into account anyone else who also pays part of your household expenses (which might include your fiancee, SNAP benefits, section 8 housing, and so on). Money that your fiancee spends on things that are not household expenses (like child support to a past relationship, tuition, loans and other personal expenses) don't count as household expenses so it is possible for the lower wage earner to be HOH depending on how the higher wage earned spends their money. Again, see page 8 of publication 501.
