My friend doesn't want me to date her brother, what should I do?

Anonymous
Long story short here are her reasons
1. She would rather us see/date other people
2. I'm her friend and he's her brother that's how it should remain
3. She would not even want us to be close friends (although I have a friendship with him already)
They have a very close relationship, I joke about it all the time with them. But I know she can be pushy in regard to who he dates.

I honestly don't think she's given valid reasons, aside from coming off as selfish (she admitted this too). But a part of me hopes that if I do become serious with her brother that it will upset her. So I need some advice.
Updates
+1 y
Also forgot to add she also said this "I'm possessive of my brother and friends so them dating is not an option."
My friend doesn't want me to date her brother, what should I do?
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