It seems that the mother was a princess in her own land, per someone mentioned? She had been shown as loving and courteous so far. Something that the father seems to not been doing after gotten the second wife. She had enough self-esteem to recognize her worth and enough resources to bail when she wasn't treated the way she should.
For me, it seems to imply that theirs was a political marriage. There seems to be no love lost between them. She would have stayed if Ains wanted to, that much was obvious. Both were unhappy enough to bail. Not a lot of people, IRL or many stories we read, had the capacities or resources to do when they wanted to.
I admired the mother for bailing, especially after confirming that her son also ok with it.