How do I talk to my short-tempered sister (who has always gotten very defensive when anyone has criticized her kids about something) about her daughters using smart phones in church?
To summarize the question/backgroung and the answer, simple and straight: don't.
From what you said, and the family background, nothing good will come out of this. In some similar situations, anytime I tried, I failed, and only annoyed family members or other people. They got angry and things quickly went south.
What you might try is to talk to the girls. Nicely, no pressure on them. Kindly inform them. Give them a chance to understand how their behaviour has an impact on others. Telling kids (especially not yours) not to do something isn't the best way to act, (m)any parent will tell you. What may work better is to titillate their sense of responsibility and grow awareness of the needs others have, boundaries, respect...and so on. So, more or less, something along the lines of this:
I wanted to talk to you girls, do you mind? A minute of your time please? No offense, but I think that, last time, in church, you bothered some people. You were using your phones, and they noticed. It would be nice if you could turn them down (the phones, not the folks :)) next time. It would be also very nice if you could think about that, it's up to you to decide what you want to do. Thanks.
If it works, good. If it doesn't work, no big deal. You tried.