Okay I remember now. She calls him both “イトコ” and “イトコの子”, but there was much discussion over the latter term. While it does literally mean “child of cousin”, it can also mean “child cousin”, as seen with
the series of that very name, where they're just cousins. As such, I think the 2 options remain, with neither definitively ruled out:
1. Yuusei is Sumire's cousin. When she says “イトコ”, she is being precise, and when she says “イトコの子”, she means young/child cousin.
2. Yuusei is Sumire's 2nd-nephew (“1st-cousin once-removed” in inferior terminology). When she says “イトコ”, she is being imprecise, and when she says “イトコの子”, she is being precise.
The description calls Yuusei her “従兄弟” [itoko], a more formal way to say male cousin. Is even that used loosely?
Also, in Chapter 1, it's established that Sumire is indebted to Yuusei's mom, so if Yuusei's mom ever shows up and is established as either Sumire's cousin or Sumire's aunt, then we'll know for 100% sure.