Leave well alone. She is coping in her own way. unless she not is feeding them there is no need to worry. Just make sure there is plenty of nesting material for the seperated youngsters.
She obviously is realising that there are some runts in the litter and is seprating them.

Sometimes the mum will abandon the smaller youngsters or kill them if they are not doing well.
Only if she does not feed them, then you can interfere and remove the abandoned from the nest with rubber gloves ,without disturbing her or the other nest and try syringe feeding an orphan formula.