If your estranged wife is accusing you of this and has gone to the police about it then it could get worse before it gets better, but has she just TOLD you she us going to the police? They will make some investigations and they have to take it seriously, but i would suggest that their mother will be looked at closly aswell , and so will any man she is in a relationship with, and so she could open a can of worms that harms so many people that your daughters will be confused.
has she approached the authorities? The police will not charge you unless the CPS is happy that the evidence is worth taking to court. If the child services feel the girls should be put into foster care because the mother is perhaps also a danger to them, then they will, has she really thought this through?