Head of household when I had a dependent half of the year and roommates whole year

My specific question is whether or not I satisfy the “paid half of the cost to keep up my home in the year”

Here is a breakdown of the events this year:

  • From Jan to Dec I have had 2 other roommates. We split rent 1/3 each from Jan to end of Aug.

  • In middle of June, my younger sister moved in (sharing my room) as my dependent. (She was definitely with me more than half the nights of the year)

  • I began paying 1/2 of the rent Sep 1st to cover my sister’s share of the rent and my own.

Do I qualify as head of household? Some posts/articles I’m reading say a room counts as a single household in this situation, so I was paying the full rent for the room the whole year. Others say it involves the entire apartment. Just not sure what constitutes as a household in a roommate situation.