My brother-in-law has power of attorney for his(and my husband's) now deceased father, there are 6 siblings, does my brother-in-law have any say in who inherits the estate and how it will be divided?

Not one bit, power of attorney ends at death, so once his father died, the power of attorney ends. Now the executor of the estate will have to give property as the will says or if no will as state law says.
So he will need to file the estate in probate court and they will appoint someone to do this ( unless someone is named in the will)