It's true, ML first started to pay attention to FL because her abusive family reminded him of his own sister who was abused to the point she got used to it. He wanted to protect her because he couldn't protect his sister. But it's apparent pretty soon in the story that there was more to it for him. In his dream, he saw his sister with Ruth because he wished she were alive to see her son growing up. But then his sister's face is replaced by FL because subconsciously he had started to see FL as his family and Ruth's mother with himself as Ruth's father. That dream is supposed to be the beginning of his acceptance of FL into his life not something weird like romanticizing FL because he thinks of his sister when he sees her.

Even later when FL asks him if he sees his sister in her, he tells her honestly that it was true in the beginning because of the abuse she suffered but soon after he saw her as her. She was different from his sister and her own person. It's not just his words, even with his actions later on it's clear that he is speaking the truth.

See, as you delve into ML's own abusive past, you see that he had absolutely no one to rely on besides his sister. She was the only compassion he had ever known, so she became the only standard for kindness and warmth in his heart. So, when he met FL and was faced with her own kindness, he subconsciously compared that to his sister. This made him more humane in my eyes and I can understand where he's coming from.

*since he didn't want to tell Ruth that he was actually just his uncle, and not the dad. He even went as far as to introduce Julia's portrait as Ruth's aunt and not his biological mother (which Im pretty disappointed about since Julia really loved Ruth even before he was born and would definitely be sad if she found out that her brother didn't even let her son recognize her as his mother).