Reddit mentions of The Choices We Made: Twenty-Five Women and Men Speak Out About Abortion

I think, depending on how you feel you could handle yourself, that asking her questions about it might be really helpful (this is assuming she does want to talk about it). If she's down and you're able to assume a position that's more about listening to her explain her rationale and feelings before, during, and after (and keeping comments down and your facial reactions neutral), it might have a positive effect on helping you feel closer to her and more understanding, even if you still feel like it was a wrong and immoral choice.

If neither of you are in a place where that conversation wouldn't turn into something undesirable, my recommendation would be to turn to someone else to bounce your feelings off of. They can sometimes be hard to get through to, but I can't recommend Exhale Pro-Voice's hotline enough- it's a place you can talk to someone about abortion, regardless of your stance on it, and they can help you to sort of untangle the emotions you're going through.

Barring that, reading stories from other women who have had abortions might help if you can't directly talk to your sister about it. If struggling to understand why she did what she did is the main issue, then gathering up that information can really aid you in processing it. There are so many individual stories and situations and reasons for abortion, but the more you absorb and take in, the more you're able to understand and empathize, even when you ultimately still disagree. It's something I recommend for everybody, pro-choice or pro-life. Some places to start:

r/abortion

1 in 3 Campaign

My Abortion, My Life

Pregnancy Choices Directory

We Testify

The Abortion Diary (really great if you like podcasts or being able to listen to women tell their stories)

The Choices We Made: Twenty-Five Women and Men Speak Out About Abortion

May Cause Love: A Memoir by Kassi Underwood