Like many children with a new sibling, DGD got lots of presents and fuss when her brother arrived. It was the period after that she regressed, so that is the time when parents need to start focussing on the older child. Perhaps they could divide and rule, while one concentrates on the baby, the other gives time and attention to the older child.
I think the main thing is to be aware is that this behaviour is understandable and treated sensitively by all, it will pass. For DGD, once her brother was sitting up and crawling and could be bossed around and played with everything sorted itself out. Now, despite a three year age difference, being different genders and completely different personalities with different interests they are a formidable team and deeply protective of each other.