The wrens

This is what my Christmas cactus looked like last November. Now, it's just as big, but no flowers and it's sitting outside on our deck.

The last two summers it was the nesting place for a pair of Carolina wrens where each year they raised 2 clutches of 4 babies. My husband and I really enjoyed watching the babies grow and the parents enjoyed the mealworms that I put out for them each morning and evening. We are hoping that they will nest here again this summer.
I had a conversation with my sister yesterday, who is so not-a-bird-person. A few weeks ago she had called to tell me that there were noises in the walls of the house they had just bought, and she thought it was birds, because she's seen a bunch hanging around.
My husband thought it was more likely squirrels or mice, but she called back today to say that it was definitely birds. She has found the hole where they are flying in (carrying nest building materials).
She discovered this by hanging her head out the window and watching the birds until they flew into the hole, and it was only due to the fact that her storm window was made of lightweight plastic rather than something more substantial, that she wasn't
beheaded when the window came slamming down on her neck like a guillotine.
Who knew that the "sport" of bird watching could be so dangerous?
Sister: "Do you think if I put moth balls in there, it would keep them out??
Me: "No, I think it would kill them - I think you should call the Cooperative Extension to see what they suggest."
Me: "What kind of birds are they?" (I don't even know why I asked this, because the chances are close to zero that she would be able to identify them unless they were something as obvious as a robins or a crows.
Sister: "I don't know" (big surprise). "You know that cartoon of that little blue bird that looks grouchy???"
Me: "No, I don't"
Sister: "Well, they look like that, except they aren't blue. They're small and short and squatty and brown."
Well, with me at one end of the line leafing through my Audobon bird book, and her at the other end of the line describing them to me as she watched them, we tentatively came to the conclusion they they are probably house wrens.
Sister: "Well, I'd better get off the phone, I have work to do"
Me: "Don't forget to call the Cooperative Extension - TODAY, before they start laying eggs. I don't want another call in 3 weeks with you asking "What am I going to do about these baby birds in my wall?"
But what I really don't understand is why she doesn't just let them live there.... I would.