What does it mean when a guy says she like a sister to me?

I'm wondering if your boyfriend says you don't have to worry about her she's like a sister to me and you find out that they slept together, she was always showing her breasts to him and that he asked her on several occasions to move in with him as well as that she had broke his heart to many times.
What do you make of that?
What does it mean when a guy says she like a sister to me?
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