Ive been in the same boat, but with my older sisters. Having once been religious yourself (as was I), you probably know the fear of allowing yourself to question your faith. I eventually had to ask my sisters to not discuss matters of faith with me, and they requested I don't present my opinions either. It honestly makes for an unwelcome distance, in my opinion, but I love them, and they love me. I completely understand the concern for my “immortal soul” that they have. Ive tried to explain it’s not a choice for me, but they can’t get their heads around that. It can be very sad and frustrating.

sounds like a discussion might be in order to negotiate no-go topics, you are both entitled to your own differences and respect. I have friends who might press some of my buttons and the only way through has been to have the talk about your relationship and not about any beliefs; so the beliefs can stay personal.

You are blood family but different ppl. take that and run with it. having lost both my siblings already i've realized that what they believed is a moot point and i would just love to have them back so i could talk to them at all... learn to agree to disagree imo.

It's hard, but if you want to maintain a relationship with your sister, avoid the subject of religion. If you're both adults, then you are one of you has as much right as the other to make her own life decisions. Nothing to be gained, but a lot can be lost, if you try to control one another.

I have 5 older sister's so I totally understand. 4 3 are atheist like me 1 says she's a believer but doesn't go to church. The last one is uber religious. We had a really big falling out at first because as my older sister it was her responsibly to save me. I bucked that like a bucking bronco.
Finally we had it out she telling me what she believes and then I told her what I believed. We realised we love each other.
So now when she says god bless me I say thank you I will tell my cat to pray for you. We both laugh.
Point being I am not going to change her mine she's not going to change mine. We have so much other stuff to talk about. We have so much shared history. We are family.
We tolerate what we see as crazy in the other. Lol
Don't give up on your sister. It would break your mothers heart to know her children aren't family anymore.

It is easy you just ignore her until she asks for advice . It took my sister 40 years and many different churches but she is now a strong atheist. We had many discussions along the way and they were very interesting.

Why does she have to understand your position? Maybe she just cant?

It made me sad to read your post. Seems a rather unequal relationship if you are willing to accept your sister's X-tianity, but she won't at least agree to disagree in return. Maybe, to help you remain in a positive space when you are together, you can get her to do just that, agree to disagree on the topics of Atheism and X-tianity and just don't go there.

I my self am in simalar circumstances with my family. They are still my family and we have other commonalities besides belief/disbelief. We find common ground, and if one of us enters in to a discussion of religion or something we disagree with, we have a mutual understanding and ask the question of "do you want to go down this road", or "can we find something we both agree on" and subject is usually brought back to a civil road. We also have a mutual understanding. Although this did not occur over night, it has taken years of work and we continue to work on it. Its not an easy task and some of my family memebers were unable to accept this and we no longer converse. Its hard but sometimes its nessicary.

She is your sister. You're always going to have her in your mind. Since you came from a Christian background, it's easier for you to understand her point of view than her understanding yours as an atheist. Since she is younger, she might need more time to accept who you are now. Be respectful, kind, and loving to her; be her sister. There's a chance she might never get there, so be prepared for that too.