I only have one sibling still active in Christian fundamentalism and he has never tried to proselytize me, unless you count highly ignorable, very lame, half-hearted, passive-aggressive hint-dropping as "proselytization".

On the same principle that you oppose proselytizing, you should not offer your opinions or advice unless she seeks them out from you. Then be honest, but kind. As to "honest but kind" your understandable anxiety is that any honest disagreement will be automatically seen as unkind and personal even though it's neither. But that would be your sister's problem, not yours.

In summation I would not choose to die upon this particular hill and just let my sister be as she is. But if she sought my opinion, she would most certainly get it.

I usually ignore them, if they become a pest about it the gloves come off. In my home, they get one warning then shown the door. It's my castle and I'm the king. Hail to the King, baby.

In Ostfriesland, we have a saying when people come to prosletyze;  we say: be careful,. We still have a bone from Saint Boniface in our cupboard."   Backstory:  The friesians living along the coast odf the North Sea were the last tribes converting to Chrtistianity in the 9th Century.  It was by force and through deals their chieftains made with Roman christian leaders.  Saint Boniface, an Irish monk was sent to the Frisians to convert them.  The Frisians did not take well to that, killed him and ate him ( that may be a myth) Therefore , the sayiing.  If you have never heard of the Frisians, read Beowulf again.  Frisians accompanied Beowulf to fight the monster Grendel.  Also, Mennonites, anabaptist christians, have their roots in Friesland.  
Lecture over.

You can hope it is a phase, and that she will move on. In the mean time you accept her and let her her do what she wants as far as her religion is concerned. If she "god bothers" you politlely, but firmly, tell her you are not interested in what she has to say, if she can't accept that, walk away. Getting into a discussion with a god botherer is rarely productive.

I guess I should have clarified a couple of things.
My sister is well aware of my atheism. We love one another, and respect our disparate beliefs, or lack thereof. If we didn't live 6000 miles apart, I would be tempted to set up a Pastafarian table next to them and ask travelers if they have been touched by the Flying Spaghetti Monster's noodley appendage. I'm that much of a smart ass.
I thank you all for your responses. I was most interested in seeing how others dealt with extremely different beliefs among family members. Guess it depends on the family.

It's her life and beliefs..let it go..

I have had similar situations with my family. Responding didn't usually work out that good. They usually forgive you though. Most the time now, I just let it go. You can't have a logical argument with someone who doesn't use logic. It's like playing a board game with no rules, the winner is whoever decided their the winner.

Let other people give her a big what for, no doubt a weary traveller will express their disgust. If she brings it up and asks what you think then tell her.

Yes I hate that...believe what you want it's a personal choice, but please don't convert me or make me explain my thoughts.

What is your inhibition against coming back with a solid counter-argument, or correction of their logic? You are dealing with people who believe in immaterial, unprovable things. Such statements should be easy pickings.

My younger brother was a youth minister. When told about that, I laughed.
I'm not hiding my atheism for anyone. I will defend it, vociferously, if challenged. You will not win that debate with me.

If my sister would do that to me, I would say" Irespect your right to believe as you choose. I demand that you do the same for me. Otherwise, you are treating me with great disrespect. I will not tolerate that. If you do not stop your proscelatizing I will be forced to minimize my contact with you. Whaich do your want -- a relationship with me or to continue the proscelatizing? You cannot have or do both."

I feel your pain - this is an actual email that I recently got from my sister :

"We might be near the end of our physical bodies but our soul goes on eternally. Jesus said, “Truly, truly, I say to you, if anyone keeps My word he will never see death.” (John 8:51). I believe Jesus always told the truth. I know that I will have a glorious body with no more pain. I trust in Him and I hope that you will too. I pray for that everyday because I love you."

I don't tell her that im an atheist because I know that would hurt her - but im tempted

Not your problem. If her Facebook activity bothers you unfriend or unfollow. I had to do that with family members myself.

Let her follow her own path, be an example of other thinking. If you must say something tell her, “it’s not something I believed in doing but be safe” ... if you go in like bang busters you have a chance of a damaging family rift. There is the possibility that the way you feel is already representative of a rift, I would let that go... whatever you do, do it with kindness, then go out drinking

This really comes down to you and how well you know your sister. Is your position on religion already known? If so, then she might not be so surprised when you tell her that you respect her right to practice any religion she chooses to believe in, but you feel that imposing her beliefs on others in any way is wrong and that you disagree with what she is doing. You could also set up a Nonbeliever's table near where she is doing her thing and print up a bunch of FFRF handouts.

I agree with you that what she's doing is wrong. I hate it when I'm at a mall and see a table with a pious-looking grandma sitting there handing out religious shit (and I do mean shit). Over the holidays, I attended an outdoor Xmas thing (I'm not a believer, but I love all the pretty lights, m'kay?) and there were a few people there standing in front of a large (like 10' tall) white cross, large home-made signs telling people to repent and come to jesus. One of them was shouting through a BULL HORN for crying out loud. I wanted to smack them. Especially bullhorn dude. Anyway, hope this helps.

As an atheist, I have sort of pussy footed around relatives who are devoted christians...but no longer...I now just say it, feel it, live it and let the shoes fall where they may.......

I am sure someone will tell her to shove it on the train platform and others may actually work to enlighten her, in other words hand back what she is passing out and see how she likes it.

My brother would have I think. We have had discussions but generally I hit him back with a clear understanding that religion is a very... VERY extremely personal and private matter. I don't push mine on him, he won't be pushing his on me. It comes down to that "you do you" and "live as let live" attitude.

As much as I hate it ignore it and if it does not affect you personally do not become involved in a family situtation. You have only one family that you did not get to choose but to contend with. Keep it as pleasant as you can ,ignore the religion and keep the peace

Your relationship is probably quite a bit different than mine with my brother because I'd just tell him to "Shut the fuck up!" most likely he'd make a smart-assed remark, but he'd stuff it because he needs me and my support more than I need him.

It is immoral to claim to know things you can't possibly know. Perhaps share that this is your perspective and encourage her to engage in a reason and kindess campaign instead.

Not religious except my mother's family. I see them on FB only so not an issue for me except family funerals