Congrats on your nice surprise. I have no idea what their weight should be. With the yolks sacs gone and it almost completely closed, they could be a little under as they may have been there longer then you think. Start giving daily warm soaks and feeding them and they should hopefully be fine
The older brothers have some trouble some months ago, I wonder if the fact that the mother is young could be a problem for the baby I hope that someone whit more data will share with us some information.