In this case you need to stop the older kittens.
The best thing would be if the mother and the young kittens could have a room for themselves, while the older kittens can not enter.
It is important the new kittens are getting enough milk and especially the early milk since it contains important antibodies and nutrients.
Spay your cat now unless you really want more kittens
As my answer to this question about timing between cat pregnancies explains, pregnancies are exhausting just like they would be for us humans. Since you already have a lot of kittens I would suggest spaying the cat so she does not keep getting pregnant.