Hi there  Just wanted to let you know I had a similar problem with my 5 1/2 year old, started about 6 months after his sister was born.    We're past it now it only lasted a few months, but I found the book 'Love Bombing' really worked and helped us both get through it.  Principle is to have a special day with them one on one time and let them make all the decisions whilst you reinforce how much you love them, like when they were a toddler.    Can work up to 12 years old.    Good luck and remember its just a phase.