Let her know you really want her approval and you really want to know exactly how she feels about the whole thing.
You don’t necessarily her permission to be with him.
But I love his sister like she’s my sister and he’s close to her as well. Things can get messy fast and it becomes really easy for relationships and friendships to get ruined.
She knows I like him and he likes me, but I don’t think she’s ever really thought about us getting together. Being with your bestie’s brother can be really awkward for everyone.
If you’re still willing to give the relationship a try, then talk to your bestie.