What happens next?

How to tell your siblings you are bisexual
Tell them in person if you can.
There are plenty of ways you can go about sharing the information you're hoping to convey to your siblings. Your relationship with them, as well as any potential qualms on their part will factor into this decision.
It may seem hard to bring up the conversation. You can ease into a more serious conversation by saying something like, " there's something important that i want to tell you because you mean a lot to me.