She was a woman of her ethnicity and her era, and subordinate to Vito, yet she had Vito's complete respect and that of her sons. Sonny goes out of his way to tell Michael he'll straighten it out with mom about his not being able to say goodbye properly. Sonny also minds her when she tells him not to interfere.
Even Michael, bas ass that he was went to her in a moment of remorse asking her whether by protectiing his family he could lose it.

And of course Michael would do no harm to Fredo as long as their mother was still alive, so she would not feel the pain of losing a second son.