Would my wife, with whom I have three children, be Haram for me, given that her younger sister was nursed by my mother? And what are the implications of that?

If the wife was not suckled by the husband's mother, and did not share suckling with her from another woman, then she is not among his mahrams, and his marriage to her is valid and not to be annulled. Her younger sister having been suckled by his mother does not render her unlawful to him.