My boyfriend takes care of a kid that's not his ?

Okay, I need advice. I'm madly in love with my boyfriend but the fact that he is taking responsibility for a kid that isn't his is driving me crazy. I just think of it this way, I believe it's not fair to me or the children we have alrdy. I have a kid from a previous relationship and so does he. I don't see why we have to take care of a child who doesn't belong to either of us. That's one more mouth to feed, extra holiday money, etc. The mother of the child is insane I just don't see why he would wanna be attached to her, the mother, if he doesn't have to. I don't understand why he wants to be totally involved w this kid when him and the mother aren't even together anymore. We also have a child on the way and I just don't think it's right for him to take care of this kid. Advice? Also am I wrong for the way I feel.
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Ok maybe I should have explained more. I treat her as if she is my own. I am not mean to her when she is w us. Her mother pulls her in and out of my boyfriends life. She still allows the child to see her real father and then pulls her away for the time being, but when she needs money or gets mad at the real father she wants my boyfriend to be dad again. That's not healthy for anyone in the situation including the child. I feel like she takes advantage of him because he has a kind heart
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And yes, he treats her better then our biological children. Also I didn't know the situation til after I was pregnant because the kid wasn't around, she came in out of nowhere.
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Yes I am mother. I would rather my daughter have just me and all my love w no dad then call someone who isn't her dad, daddy
My boyfriend takes care of a kid that's not his ?
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