I agree with Knotted Yet. I'd tell my sisters. That may be just me, my sister and my husband were the only two that knew about my cancer for a long time. My mother was the last one I told, but that's my family dynamics.

If you do tell your sisters, order - don't politely suggest - order them to have fun with you on the upcoming ride. The pity party can happen afterwards. The bike ride will be for mental health. There will be more than enough time for a pity party.