What crucial role in the creature's development is played by the DeLacey family?

He learns the power of speech by listening to the family from afar. This was the most important thing he learned from the family. He also saw them as his family, he learned from seeing their happiness in each other even as he saw them facing adversities like poverty. Because of watching the family, the creature realized that possibly the thing that would make him happy was a companion or mate with which to share his life..... thus, his request later in the novel to Frankenstein for a mate.