I guess her mom was already fed up with the situation, how her husband mistreats her, don't value her, does not respect her so maybe she thought of it carefully if she will stay she willmloose her sanity plus with the baby it will be quite a handful for her to bear. it would be hard though to think she left her daughter. Probably she knows she cannot provide for her.
That's the problem when you just jump right away, she was young just sixteen then having to deal with an insolent, irresponsible husband was too much for there to bear.

Happy to know that your niece was loved and that she grew up to be a pretty young lady full of values.
Hope one day she will meet her mom and settle all the differences and explain to her why she did what she did.
Time can only tell what will happen next.
Loved thr story you shared to us sis. We can learn something from it.
Best of luck to your family and God speed always.