Can kittens drink from another mother?

Hye, i have 4 kittens from the same mother. I also have another cat, 8 months that is pregnant. She was the first batch of the mother cat. Today, i saw that the kittens ignored their original mother and drink milk from the 8 months cat. it is normal? i cried when i saw this because the original mother tried to call to her kittens but they go to the other cat for milk. what should i do?