Is there a name for when a woman leaves her husband to be with her kids?

Divorce?

I don’t think there’s an official term, so you can make one up. Change in priorities complex? Interfering grandmother complex? I don’t give a shit; I’ll do as I damn please complex?

In some cultures, it’s normal for grandparents (particularly grandmothers) to live with their grown children and take care of their grandchildren.

Except for the intimacy, sex, rapport and/or involvement part, she has empty nest syndrome.

I don’t think this is a complex or a syndrome. I don’t think it’s all that common.

If she moved in with her son and his family, there might be a bit of an Oedipal complex there, especially if she tries to usurp the wife’s role in the family. But more than likely it’s probably empty nest syndrome. She’s not ready to stop parenting, possibly because the relationship with her husband died long ago and she depended on her children to give her meaning in life.

Lonelydragon is correct. It appears she has fallen “out of love” with her husband.

The end solution, in this case, is usually divorce.