This has happened a few times recently with my SIL when I've been dealing with my 4 year old. Yesterday DS1 was having a tantrum about not wanting to do something. Eventually I sat him on the step and was about to explain to him why I'd put him there when SIL came along sat next to him and cuddled him and said "Now DS1 how would you like to come out for the day with us tomorrow? If you do as your told now you can come"
To which obviously DS1 was delighted and stated "great but not with mummy she's horrible"
To say I was pissed off is a massive understatement. This isn't a regular occurance as such but it's happened more than once and I want to know how to deal with it at the time if it happens again. I admit me and SIL have VERY different parenting styles and that I'm sure she genuinely thought she was being helpful but I felt undermined and made to look like an horrible ogre.