Surrogate mother mice

Question
Is it the norm that other females, without babies, will suckle the babies of other mice?

They weaned their babies about 2/3 weeks before these new ones were born.  They are so protective of these babies as well.  I get warning teeth put on my fingers when Im fiddling ( can't call it biting, its too gently done)

After such a long time, would their milk start flowing again? Or are they just being used like a pacifier/dummy?

This is not the first time this has happened with my mice but the last time there was only a short period between the time they weaned theirs kids and the new ones arrived.

The only mouse I never saw suckling the babies was my sterile female mouse, who I had to have put down today because her cancer tumors got too big.

Answer
Dear Sandra,

I have actually even encountered a mouse who had never been a mother nurse the babies of a mother mouse who died.  It's pretty amazing.  Because mice get killed often and because mice in the same nest tend to be close relatives and therefore carry many of the same genes, it is evolutionarily sound for a female mouse to nurse other babies in the nest.  

Squeaks n giggles,

Natasha