Can I claim my 3 foster kids ( who are my nieces ) that I have had custody since 7/5/19, but the foster mom claimed them on the 2019 taxes? Is their a split ?

Is it only who has them longer or is it 180 in my custody?   
Also won’t this effect the stimulus payment of 500 for each child when they are in my custody ( by court order ) ,  not the foster mom who is no longer involved?