How is Aunt Alexandra a good mother figure to Scout?

Aunt Alexandra is highly critical of how her brother raises his children. In her opinion, he lets them run wild; he doesn't set appropriate boundaries for their behavior; he teaches them the wrong values; all in all, he isn't doing a very good job of bringing them up properly. Alexandra is especially scathing of how Atticus has raised Scout. Atticus's sister has a rather old-fashioned understanding of how women and girls should behave. She sees herself as a fine, upstanding Southern lady and wants Scout to grow up to be just like her. But as Scout's such a tomboy, always running wild and getting into scrapes and adventures with the boys, that seems like a pretty tall order.
On the whole, Aunt Alexandra is an inadequate stand-in mother for Scout and Jem, but in some ways she supplies the deficiencies of her brother's parenting skills. Setting boundaries for children is very important, and no one needs to tell that to Alexandra. The problem is that the boundaries she sets are a little too restrictive; they don't allow Scout and Jem to go out into the world and explore and discover new things about themselves and their surroundings.