Everyone thinks they don’t know someone who has had an abortion, but 1 in 3 women will have had an abortion at some point in their lives. The reason you think you don’t know someone who has had one, is because like your sister, they keep it a private matter.
I’ve had 2 abortions, and only my mom, sister, and husband (who went with me to both, and fully supported my decision) know about it. Clearly, this is something she doesn’t want you all to know for some reason (probably because she knew your mom would say something like “has a grandchild she will never see”). She is 17, one year away from being a legal adult. She is not a 13 year old girl. Clearly, she managed to get herself a safe procedure at a clinic (for which you should be grateful we live in a country that she could do so). You should respect what is now her private, medical matter, and not say anything unless she chooses to tell you.
Despite what most people believe, the vast majority of women who choose abortion do not have mental breakdowns from the decision (again, do you see 1 in 3 women going crazy with grief everyday?). Most feel relief afterward, and I know that if I had been a 17 year old girl (and not a happily married woman) when I had an unplanned pregnancy, I’d probably also feel relief knowing that I was able to handle the situation myself, and not have to upset my entire family with the choice I made for myself. Unless you see anything to indicate otherwise, she is probably okay with her choice, like most women are.