My cats Mother wants to help

I have 2 grown cats. Izzy is the mother of Phoebe. Izzy had 4 kittens 5 weeks ago. Phoebe just had 4 kittens today. I have Phoebe in my bedroom in a large plastic tote. These are Phoebe's first litter. Whenever the babies start fussing loudly, Izzy leaves her babies and jumps in the box with Phoebe and her babies. She licks and lays with them and is even nursing them along side Phoebe. I know you can put abandoned kittens with a nursing cat and she will take care of them, but I am worried that jumping from one litter to another, might make the newborns sick. Should I make Izzy stay out of the box? Phoebe seems happy to have her there. Bty... Phoebe has been letting Izzy's kittens suck on her for a few days now. Not sure what I should do.

If Phoebe is alright with it then my suggestion is let her

It quite normal for mother cats to share the work and nurse each other kittens

Thank you for the quick reply! Didn't know that was a common thing. I can relax a bit now! Phoebe is the Siamese.

You may see her babysit them

If you have any more questions please ask.