The first option would have been to rub the lambs with the birth fluid of the single that died, but this all happened too late for that.
The ewe needs to believe the lambs are hers. Most sheep ID is by smell, a bit by sight and sound. To confuse the ewe, it can help to rub some of her expressed milk onto the lambs' heads and faces, and around the backside. Express some of her milk (sit her on her rump to do this, and use a lubricant). Collect in a bottle and feed this to the lambs so the smell of faeces made from her milk soon comes through them. This is better than reconstituted milk powder.
She will be getting used to their sight and calls by them being in the pen with her.
There is a danger she will kick them, so make an area across one corner of the pen, from narrow planks with gaps, which the lambs can squeeze under but she can't reach them.
The last resort is to tie her up for a couple of days, but I don't like this option.
The dead ewe may have been in need of emergency Calciject.
Did you dip the cords in 10% iodine as soon as the lambs were born?
Are your bottle teats perhaps cow size? It's worth buying the sheep version.
Things will improve