First Nephi 7:5

And it came to pass that the Lord did soften the heart of Ishmael, and also his household, insomuch that they took their journey with us down into the wilderness to the tent of our father.

The Lord did not override Ishmael's agency or force or coerce him. He softened his heart by testifying to him through the Holy Ghost that it was God's will that Ishmael join his family to Lehi's. It was really Ishmael who softened his own heart. Soft-heartedness is a gift as well as a talent.