She was seen as more of a disciplinarian than her husband and got tired of him undermining her authority. She wore an arm sling due to a car accident two years prior. She explained that the pain in her arm and back made it difficult for her to follow through with discipline.
Nina was also frustrated with Paul constantly undermining her authority. For example, after she struggled to get a diaper on Gabrielle and finally succeeded, Paul intervened and just allowed Gabrielle to take the diaper off. Nina thought this was "taking away from her accomplishment."
She was also mean to Jo.
She was 27 years old at the time and is now 39 in 2018.
Nina's Facebook page We now see that Nina and Paul have divorced. She also has full custody of her kids.