When is a mother considered a wet nurse, and are there conditions for this, such as a specific number of sucklings or a certain amount of milk, and do all the siblings of the infant become siblings to the children of the wet nurse?

The amount of suckling that establishes mahramiyyah (prohibition of marriage) is five full sucklings that satisfy the child, occurring before the completion of two years of age.

What is considered a "suckling" is determined by custom: when the infant takes the breast and sucks from it, then leaves it, turning away from it.

The suckling child becomes a son to the wet nurse and to her husband, the owner of the milk, and becomes a brother to all of her children. As for the siblings of the suckling child, there is no mahramiyyah between them and the wet nurse or her children.