Listen, I'll explain it very clearly.
1. Roselia is someone who didn't really have a complex interpersonal background before. She desired strongly a family, and she didn't have the opportunity to have one until now. She was focused on her job, on the children, and that's what she is accustomed to. Not to any of this romance mumbo jumbo.
2. While it is true that she now fully lives as herself inside the novel, she still carries some notions about the characters in it. She still feels somewhat distinct from them, at a subconscious level. This "omniscient reader viewpoint" (GO READ IT BTW) subtly separates her from them, so she needs juuust a bit of time to slowly break down this line and understand that she can also love Cassius, and not only be Ain's mother. With her personality, the first one is infinitely more difficult to realize.
3. She is a dumbfuck.
AND THAT IS ALL!