{{{Marni}}}
I don't have a whole lot to add here, either, except for my love and support in whatever you choose to do.
You know your family dynamics better than any of us could hope to, but I'm with those who suggested telling your sisters before the bike ride -- maybe not today because you're still processing your own feelings and making plans of what to do next -- but I wouldn't put it off too long. If you have a pact, I'd honor that, but on your own terms. Tell them when you're ready.
If you tell them sooner, though, I agree that they'd have time to process their own feelings before the bike ride. I think you're incredibly brave for going through with the bike ride at all.
Vent away. Dither dither dither. As someone else said, you have a lot of friendly shoulders here to lean on.
Much, much love, Marni.
Roxy
Getting in touch with my inner try-athlete.