I think a few posters have been unnecessarily harsh on OP. This is not about flowers for the MIL. If it hadn't been the flowers it would have been something else. I don't think it's any coincidence that this happened at the moment you joined their family.
I also think a personality disorder may be to blame. My own father has Borderline Personality Disorder and is extremely erratic and controlling. Growing up in such a home has left me (and all 5 of my siblings) with our own host of issues. He has frequent "tantrums" (we call them) that he throws in order to control us. The situation with your MIL was SO something he would do.
This book really helped me: http://www.amazon.com/Stop-Walking-E...6468460&sr=8-1

I also think a personality disorder may be to blame. My own father has Borderline Personality Disorder and is extremely erratic and controlling. Growing up in such a home has left me (and all 5 of my siblings) with our own host of issues. He has frequent "tantrums" (we call them) that he throws in order to control us. The situation with your MIL was SO something he would do.
This book really helped me: http://www.amazon.com/Stop-Walking-E...6468460&sr=8-1









