The girl best friend doesn't like me. How do I stop her from ruining a possible relationship?

The title sounds bad, I know. But please hear me out.

This guy is my best friends brother. I really like him and everyone around us (including my best friend) are very supportive and thinks he likes me back. But I don't think his friend (a girl) is. (They also dated for like 3 weeks when they were 16 but that was around 3 years ago now. They have also known eachother longer than I have)

I'm not a jealous person, and I don't even know her but I know she has it in for the girlfriends of her friends. For example, another close mutual friend of the boy I like and her recently has a new girlfriend. I've met her once, and she seems really nice. But the girl best friend according to the boy I like and his mum she spoke horribly about the girlfriend and mutal friend. I'm really worried that this will happen to me. And this might be me overthinking, but when I looked on her fb and insta page a little while ago (not out of spite, curiosity) she no pictures of the boy I like or the other boy with the girlfriend she spoke horribly about. And when she did, she posted them with a private audiance on facebook.

Just a few weeks ago, she posted lots of them with captions like "happy bday _ I love you ❤❤"
I don't want to fight her, and the people I have spoken to say she's not a threat. I don't want to think of her as a threat. I'm worried she'll damage the possible relationship me and this boy can have. He's been a great friend to me of 4 or five years. And now we're getting older, it's getting a bit petty thinking of fighting over him. Is there anything I can do to calm my nerves or preventing this from happening?
The girl best friend doesn't like me. How do I stop her from ruining a possible relationship?
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