How I wish my SIL could act like your SIL when the children explode. It sounds like an excellent approach to quietly but firmly remove them from the room until they are ready to rejoin the more civilised beings!
Lilikemaho, the fact that your children behaved in a different manner to your children doesn't mean that they were better than the child who finds his frustration difficult to deal with. What would you expect his parents to do? Thrash his frustration out of him? You were lucky your children were less frustrated. All my children were brought up in the same way but one of them was always more frustrated than the others. She always wanted to do more than she could developmentally and so would get frustrated. She was totally unable to express herself and it looked like she was spoiled but she certainly wasn't.
What is a reasonable minimum spend for an online grocery delivery??