My sister refuses to commemorate my parents 50th wedding anniversary.

After my brother's death two years ago, I was expecting my sister and I to be closer. We have both worked at this but now I've stopped trying to talk to her at all. Our parents 50th wedding anniversary is coming up. I asked for her contribution (for a gift, sign the card, help w/money etc) My sister refused saying she could not in good conscience commemorate our parents anniversary because their marriage has so many issues and is a sham. I've yet to meet anyone who's been married more than 10 days that doesn't have issues and my parents are not abusive, they are loving people who walk in all the light they have. They are human and have always been there for us! I told my sister that our parents are in their 11th hour and (not 4 years ago she and her husband on their 25th anniv were talking about divorce) so why not let whatever issues you have go, love and go on? Love is far more important in life and in the end is all that matters. Now with elderly parents and a horrible economy, if/when something happens with my family I will need her help but don't feel she is going to or willing to help any longer. I feel abandoned by her decision (could not and will not tell my parents - they'd be devastated).