I am so sorry you are going through this. I guess statistically 1 out of 9 or 10 women are affected. Once you include their families and friends, at this point I think everyone knows someone in your position.

I have 4 sisters, we are fairly close, and yes, I would struggle with this decision as well. Much as I might want to keep things to myself, I just can't see any benefit by not telling them if I had any kind of a serious condition.

Since you are close with your sisters, they will notice changes in your attitudes and behavior, or preoccupation and I would guess they will be wondering what's going on. It will be very difficult to even have phone conversations over the next 2 months and have them not notice even subtle changes or that you are not as forthcoming as you usually are.

I know my sisters would be very hurt if I were to withhold this news for so long. Your bike ride is not for 2 months - telling them now lets the news sink in, and gives them time to think about it and to react. I would expect that sharing this will bring you all even closer and make for a better bike ride when you do all get together.