I don't think the acts of texting or phoning are cheating, but if the person texting is secretive about it and isn't willing to show the content of the texts then there is something else going on. I'm racking my brain tying to think of any logical reason that would justify this activity, but I can't come up with anything. Ffield service is not something that should be secretive and that would need so many texts and phone calls everyday.
There was a recent study, either in a WT or in the God's Love bible study book, that addressed the issue of a married man or woman spending too much time or talking too much with a member of the opposite sex, and how that would be inappropriate behavior even if the intentions were to console or encourage the other person. If a sister is having a hard time with something, she should not be calling an elder's home or texting him to speak with him about her personal matters. That is disrespectful to the wife and just makes the sister look like a floozy. The same applies to a brother.
Your BIL sounds like he's up to something fishy, and your sister needs to confront him about it. If she is too scared to do anything then she's going to be stuck in this mess until he ends up getting caught in some risky business by someone else, or he just up and leaves yoru sister one day for the other woman.
If anyone can get some concrete evidence of this then I would suggest doing that asap! Aren't there ways to get records of the content of text messages that are sent and received by your phone? You sister really just needs to get the nerve to grab his cell phone and read the text messages. Anything other than field service related material should be forwarded to her cell phone and the elder's cell phone!