Thanks for submitting your question to Alterheros. It sounds like you’re in a frustrating situation. Hopefully this can help you sort out what you want to do. I hear you say that you have a close relationship with your sister, but she doesn’t know you are bi nor do you think she would approve of it. On top of that, you are in love with her best friend’s little sister, and have hooked up with her before, but are unsure of whether she wants to date you. So, before you worry about your sister or your sister’s friend, you should talk to Magan first to see if she feels the same way as you. Something as simple as, » hey, I think you’re a really cool person, and I’d love to get to know you more because I like you a lot. what do you say? » That should encourage her to share any reciprocal feelings if she has them, or decline your offer if your hook up was a one time thing in her eyes. So answer that question first before you do anything else. If Magan does not feel the same, then I think you can wait until you feel more ready to tell your sister.
In the event that Magan feels the same, I would then talk to her about the best way to broach this with your sisters. It would be a good idea to tell your sister together, and tell her sister together. Or even, the four of you together. The united front you would have with Magan should make it less nerve-wracking than by yourself in this particular situation. Also, ask Magan how her sister would react. It would be very helpful if her sister was supportive, that way you have another person there to advocate for you both in front of your sister- someone who is older and that your sister may listen to more intently.
As for yourself, and telling your sister, remember that your sister loves you. She may not like it initially, but hopefully with time, will try to understand and support you eventually because she loves you. I understand your fear about losing that closeness with your sister: being so close is a truly special thing. However, your bisexuality is a part of you, and something she must accept and support if she is to remain close to you. Friendships and being family are not about picking and choosing the parts of a person you like and discarding the ones you don’t – its a package deal. I’m not saying it will be easy, but it would bring your relationship to deeper level- getting closer to someone isn’t always a pleasurable experience. In fact, enduring and sticking by each other through rough patches and hard times garners a relationship with a very deep bond that you don’t get with someone you’ve only been through good experiences with. Either way, if you do not eventually tell her, you will probably not feel as close to here with time. Ultimately, telling her or not telling her, could potentially distance your relationship- its just a matter of whose uncomfortable (only you, if you don’t mention anything) or both of you (if you do tell her). I think the latter is better, so that all of the emotional stress that arises from this situation isn’t solely bourne by yourself.
On the other side of things, you may be surprised at her reaction, perhaps she is very supportive, especially when you are all in such tight knit relationships. Sometimes people grow more soft and understanding of a situation when it is relevant to themselves – in this case, you are her sister, and in a healthy sister-sister relationship she loves you and wants you to be happy and treated well.
I hope that has helped you somewhat, and if you have any more questions, please don’t hesitate to ask.