Trying to come out to friends & family - advice?

Hi I'm sure that (of course) you don't want to hear about every single detail, so I'll try and keep this short:

I'm a 14 year old girl and I've known for quite a while now that I'm bisexual. Ever since I was little I felt attracted to both boys and girls, but I never really talked about crushes or any of that with any members of my family (siblingswise, I have an older sister, older brother and a younger sister). I'd known what being bi meant for years, but it took me quite a long time for the realisation that I was bisexual to hit me. After a while of keeping the "secret" I began to realise that bottling it up inside of me really wasn't the best thing to do at all, but I'm having real problems telling anybody about my sexuality.

First of all, I don't want any of you to get the wrong impression about my family - they're wonderful people, and they at least appear to be supportive. It's just that me telling them I'm bi would change their whole view of me (plus, I'm really not sure how my grandmother, who is still a little bit "stuck" in her generation would take it) and they might start acting differently towards me. I guess my biggest fear is them thinking that I'm odd or different from the rest of them just because of my sexuality.
(I've explained to my little sister that I've had a crush on a girl before, but I needed to explain the meaning of "bisexual" to her as well, so she's probably forgotten all about it. She's only seven years old, though.)

The person I'd like to come out to most is my older sister, I guess. She's the person that I have the most faith in when it comes to keeping the secret until I'm ready to tell everyone else. The only problem is we have a sort of joke about me and this girl that I like (we joke about me fancying her because she thinks that there's no way that I can), so she might realise that I like her. As you can see, I've kind of dug a hole for myself here. I'd like her to know that I'm bi, yes, but not about my love life.

With regards to my friends... after much deliberation, a month ago I told (what is now) an ex-friend (we were really close at the time though), merely because she'd thought at the time that she was bi, too. However, she declared herself to be straight a week later, and turned on me the next week later due to other issues. This really struck a blow to my confidence and I can't get over the thought that she did it partly because I told her I was bisexual.

This is where the advice bit comes in... I have absolutely nowhere to turn to, and I've been looking for a forum like this for a while. I know this probably all seems very dramatic and that I haven't really got it as bad as many others, but I'm having a really hard time with this here. My family thinks I'm 100% straight and it's so fustrating not being able to tell them. If anyone has any tips, anything at all, even something that would just enable me to hint at my sexuality to them, that would be greatly appreciated!

(Sorry for all the text, and thank you for reading all the way to the end )