How do I tell my sister?

You still haven’t decided for yourself, so you’re not keeping anything from her by not telling her. If you think that she’ll react weirdly about it, you might be right or wrong – but it more likely indicates that you’re still not comfortable with the whole issue yourself.

There’s nothing, really, that your sister can offer you in terms of perspective on this that will help you figure out anything. If you feel like you can’t keep it in, don’t. But don’t feel like you need to talk to her or get her advice unless she went through the same thing.

If she did, take it as it is – her personal experience, which may confirm some of the feelings you have. but will not be able to tell you how you do or should feel.

Don’t keep it a secret; not from her. You said you guys know everything about each other – why exclude her from this? Best friends don’t judge. This will also be a good step in coming out to your family if you do discover that you are bisexual or gay. Having that support there automatically will help you a ton.

I wouldn’t worry about how to bring it up. Just wait until the two of you have some time to talk – don’t tell her as she’s on her way to class or work. Go for a coffee or something, and just tell her that you have something you want to talk to her about. She might think it’s “weird” at first, but if you can’t talk to your best friend (whether she’s your sister or not), then you just might not be ready to talk about it with anyone.

Good luck.

Edit: So I’ve read @iamthemob‘s answer which was posted a second before mine. And I agree there too. Like I said, if you’re not ready to talk about it with her, you might not be ready to talk about it period.

Only you know if you trust your sister with information like this.
Personally, my sisters are the only people in my family that know that I have dated both men and women, and neither of them even batted an eye at the news. But no one can tell you how your sister will react. You know her, and if you are as close as you seem to be, you will know whether or not this is something you want her to know about you. It sounds like you want to talk to her – and if she is the person you trust most with your deepest secrets, then I think you’ve chosen the right person, when you are really ready to talk about it.

If you and your sister are as close as you say, asking and discussing your situation should also not be a problem. if you cannot discuss this with your parents, then your sister is next in line in your family tree. i only hope that both of you are mature enough to discuss this like grown adults. this can be a serious problem for you now and in the future. by asking and discussing this with your sister, you will not only receive the truth from her, but she will give you an insight on how to handle this, from a woman perspective.

You are fortunate to have a sister that you can confide in.

This site has some great help for coming out:

http://www.hrc.org/issues/coming_out.asp

I suspect you do need a confidant in this. By definition of a confidant you need to trust in this person as @TheOnlyNeffie also states. If your sister is this, then lay it out as you are here. Find a place, space and time that will allow for this without interuption.

You just need to be prepared for any reaction your sister may have.

Is it possible that she will freak out and run and tell your mother?

If so, are you OK with that?

Is it possible she will try to talk you out of your sexuality?

Are you OK dealing with that?

You just need to know for certain how you plan to act in the face of her reaction if it is not just open arms “I love you, Sis.”

Pick a quiet moment and just ask “are there any girls that you fancy?” and see how she reacts. It is great having someone you can trust and you can share everything with.

If you think you’re bisexual, and you’re not sure, then you really have nothing to “come out” about yet. I would start by asking her if she knows anyone that’s attracted to women and men, ask her how she thinks someone would know if they were bisexual, ask her if she’s ever wondered what it would be like to be attracted to women. Eventually she’s going to clue in and ask you if you’re attracted to women, and then you can discuss what you’re questioning. It sounds like you need her input in the questioning process.

If you’re as close as you say she probably suspects ready anyway.

Another damn you auto correct moment. Changed “already” to just plain “ready.” aaarggggg!