Nanny is Janie’s grandmother and the only parent she’s ever known. We have to give Nanny props: she loves Janie and raises the girl as best as she can.
But, she's not 100 percent perfect: Nanny’s hopes for Janie’s future are skewed by her own traumatic experiences. Nanny herself lived through slavery and had a child (Janie’s mother) by her white master. Horrific, right? But, here's the thing: although what the master did was awful, it was Nanny's reputation that was tarnished—she was seen only as an unmarried mom and not as the victim of a coercive, rapey relationship.
And, to make matters worse, Nanny's daughter (Janie's mom) was raped, gave birth, and then became an alcoholic in order to dull the pain.
As a result, Nanny has high hopes that Janie will marry and be a legitimate wife. And...that's it. That's the sum of her hopes for Janie.
This goal seems to be the most important thing for Nanny, and in her quest to get Janie married to a suitable man, Nanny turns a blind eye to Janie’s needs. Nanny’s conception of freedom is one of middle-class values and financial stability. She can't achieve this goal in her lifetime, so she imposes it on Janie by making her marry a respected land-owning farmer.
And this makes Janie miserable:
[Nanny:] "Well, if he do all dat whut you come in heah wid uh face long as mah arm for?"
"Cause you told me Ah wuz gointer love him, and, and Ah don’t. Maybe if somebody was to tell me how, Ah could do it."
"You come heah wid yo’ mouf full uh foolishness on uh busy day. Heah you got uh prop tuh lean on all yo’ bawn days, and big protection, and everybody got tuh tip dey hat tuh you and call you Mis’ Killicks, and you come worryin’ me ‘bout love." (3.17-19)
Nanny’s love for Janie does not absolve her of her crime—forcing Janie into a loveless marriage and lying about how marriage leads to love. Janie condemns Nanny for this:
She hated her grandmother and had hidden it from herself all these years under a cloak of pity. She had been getting ready for her great journey to the horizons in search of people; it was important to all the world that she should find them and they find her. But she had been whipped like a cur dog, and run off down a back road after things. It was all according to the way you see things. Some people could look at a mud-puddle and see an ocean with ships. But Nanny belonged to that other kind that loved to deal in scraps. Here Nanny had taken the biggest thing God ever made, the horizon—for no matter how far a person can go the horizon is still way beyond you—and pinched it in to such a little bit of a thing that she could tie it about her granddaughter’s neck tight enough to choke her. She hated the old woman who had twisted her so in the name of love. (9.4)
By willingly deceiving Janie about the nature of love—simply to satisfy her desire to see Janie living a proud life—Nanny commits a crime that neither Janie nor God (it's implied) can forgive.