Good grief, your sil also texted the entire time? No wonder her children think this is acceptable.
Sorry, but I would have said something about modern day etiquette with hand held devices. I do not know why parents have not spoken to their kids about this.
They are to be treated just as if they were telephones attached to the wall, especially with company.
If you are at the movies, they tell you no texting! They say silence your phones and put them away.
When you are with company , you do not talk on the phone in front of them. At meals? No way, they must stay on silent.
No meals should be interrupted by the phone. You text that you will not be available for at least an hour. You can contact them afterwards.. Even if someone is sending you messages.
Phones can be left on silent and allowed to just vibrate, until a private moment.
These kids know In school, if you pull out that phone in class, it will be confiscated, sent to the office and the parent will be called to pick it up.
Your sil has done a disservice to her kids. I always remind my employees, no cell phones are allowed while you are on my time.