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I'm taking you mean your brother married a girl, and you want to date that girl's sister, right. In reality, she's just any other girl, but when things go south and you break up, things tend to get awkward.
So I don't know, you could maybe talk about it with your brother and his wife first, but you don't really have to.
If both of you are mature enough to handle this relationship, I'd say go for it.