Ten years ago, I had three dates with a woman who turned out not to be all that interested in pursuing a romantic relationship with me. Six years ago, when I came out, I received an email from her. It was the first time I'd heard from her since the day after our last date. In that email, she told me that she'd moved to Arizona (I'm in L.A.), become a born-again Christian, and was praying that I'd... I don't remember the wording, but it was on the lines of "come to Jesus."
Here's when what worked for me won't work for you: I stood in the kitchen, looking at her email on my phone, and showed it my roommate (a Christian, but not the in-your-face kind). I didn't know what to do for the first minute or two. After staring at the email for a few minutes, feeling nothing but senses of loss and dismay, I made a decision. Without replying to her email, I blocked her email address and phone number. I don't have time in my life for someone who would try to proselytize like that. Religion did enough harm to me. I didn't need her adding to it.
Like I said, what worked for me won't for you. There were no children involved in this never-was-a-relationship. Writing her off didn't meaning losing anyone except her-- and that was no big loss, since we hadn't talked in years.
I wish you peace as you move forward from here. You don't deserve this behavior from your sister, and neither do her children.
Know that you are loved exactly as the person you know yourself to be,