My daughter hasn't seen her two children or even called in over 6months, my mother and sister have had the two (1 and 3 yr old.) for 6 + months with no contact from the mother.
Can my mother and sister file for custody of the baby's,

Not custody, at least not at this point, but certainly guardianship.

But where is the father? If he puts in a claim for custody, there is a strong possibility he will get it.