Hi
First off, it is great that your sister could come out to you and your parents - that says a lot about your past and current relationship with her.
I, too, was raised very Catholic and come from a very large Catholic family. Discovering that I was a lesbian was just as much of a shock for me as if was my mom and brothers
My mom - who goes to church every day - was amazing. She cried when I told her. Later, when we discussed why she cried, she said she was mourning what she considered "losses" - that I may have a harder life, that I would never "walk down the isle", that my life as a Catholic would change and that, basically, I would be different than her. Why is my mom so amazing? Because she realized and acknowledged very early on that these were her issues - and not mine - to take on and work through.
It's now 9 years later and my partner and I are still together (and expecting a baby in 2.5 months). My mom respects my life, loves my partner as second daughter, and continues her faith. Would she choose this life for me? Honestly, probably not. Does she love me? Yes. Does she want me to be happy, yes. Has she made peace w/ her faith? Yes.
So, what can you do? Sit with the idea. Give yourself time. Find support where you need it in good friends/family- talk to others in your local chapter of PFLAG (Parents and Friends of Lesbains and Gays) b/c they have been where you've been, talk to folks in your local chapter of Dignity (the Catholic gay and lesbian group) and work through your feelings independent of your sister.
Good Luck. It will get easier. New things are always harder.
Kelly