This might be dicey - I am assuming that the mamma's chick is spending part of the day out running around. The new chicks are how old exactly?
This is how I would try it. Make sure the new chicks eat feed and have a drink of water. Put them out side for 10-15 minutes so that they are a little chilled. The mother/chick bond is a two way street, and both have to do their part. Many people put warm chicks under the broody, the chicks never having been under a broody, and the chicks come out, and the mamma can smell they are not hers and attacks them.
By letting those chicks get a bit cold (it won't kill them) Then when you put them in a warm spot, they are going to stick like a tick to that spot. They will stay there all night in the dark, and when they wake up they will be printed on her, and they will smell like her, and she will take them.
I wait until it is pretty dark, I want the chicks peeping madly in distress, wearing a sweat shirt in case she pecks you, and stick them under her. It should quickly get quiet, the mama might growl a little bit, shift around. But once it is quiet, leave them.
What worries me, is the older chick..that chick might be aggressive. But I think I would try it.
Mrs K