Damn. Just binged the whole thing, and man was that a rollercoaster. Thank you so much for introducing me to this manga! Thank you for translating. I hope things turn for the better for you soon.
My money is on it being their sister or cousin. We already seen their manager back in Chapter 16 and they look nothing alike.
Not to mention there does seem to be a resemblance between the two of them so with that, already seeing the manager in a previous chapter and common cliche's the sister is the safest bet here.