Schedule around her. Make plans to go places you know she won’t be or go places during time when she’s busy. If she’s your best friend and he’s her brother you both probably know her schedule pretty well, so you should be able to find time to hang out without her knowing. Go out while she’s at her tutor, her piano lesson, stuck at home doing homework, whatever, just work around her.
Never forget about her and try to include her. Always make sure you are spending the same amount of time with her as you were before you and her brother got in a romantic relationship. Girlfriends before boyfriends, as the saying goes. In the end you and her brother will probably end up breaking up at some point down the road, so always value her feelings. She should be top priority, no matter what.
Spend time as the three of you. This might seem awkward at first but it’s a good way to make sure that both of them feel important and included. Keep the air between you and her brother completely platonic while she’s around but also treat it like you’re on a more casual date. You can still talk to him or challenge him or do whatever you would do while your friend’s not around while still just making it seem like you two are only friends. And remember, if she’s your best friend and he’s just her brother she should always feel more important than him.
Ask for guidance. This is the hardest thing in the world, but consider asking another trusted friend, a sibling or a parent what they think you should do. If you get serious with this boy, then you can’t keep it secret forever. Ask them what they think the best time to tell her would be, how you should do it without hurting her and how long they think you and her brother can last in secret.
Don’t throw yourself on him whenever she’s not around and take it slowly. Let,s say you’re hanging out with her at her house and her brother walks into the room. She leaves to go to the bathroom and suddenly he’s right in front of you, trying to kiss you. You need to push him away. She’s only in the bathroom for a short time and you came here to hang out with her, not get serious with him.
Consider telling her. In the end, if you really like this boy and you want to take the relationship more seriously, you’re going to have to tell her. When it comes down to it, she’s going to find out at some point anyway, and it’s better for her to hear it from you than from anyone else. Keeping the relationship secret only puts stress on you and the boy and on your friendship with the girl, and the longer you keep it secret the more painful it will be when she finds out.
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