How can one ensure that the baby girl has suckled five full, satisfying breastfeeds, and do the siblings of a child who has been fully breastfed by a foreign woman become as sons and daughters to that woman through suckling, and as siblings to her children through suckling?

The nature of a satiating suckle is that the boy nurses from the breast until he leaves it of his own accord. If this occurs five times, then the prohibitive suckling is complete. If the boy spits out the breast and then immediately returns to it, or moves from one breast to another, or pauses suckling to breathe or rest, or if a light sleep intervenes, all of that is considered a single suckle. The suckling of a child from a woman is specific to that child, excluding their siblings. So, if you breastfeed your niece, she becomes your daughter through suckling and is a mahram (definition) to your children, while her brothers and sisters do not become mahrams to your children through this suckling.