It wasn't clear to me what happened to her. She had a stroke and near the end of the final episode Magda is talking to Miranda and Miranda looks very sad. She says something like "Don't tell Steve, he'll just get upset". ??? What happened?
shareSteve's mother, Mary, suffered a stroke, which left her with some form of dementia (I don't think they stated exactly what the stroke did, it just left her pretty messed up mentally). So, Miranda and Steve invited Mary to come live with them. Steve wasn't home, and Miranda and Charlotte were chatting in the kitchen, and Mary said she wanted to go to the zoo. Miranda kind of waved her off, thinking Mary wasn't serious (obviously, because Mary was mentally pretty messed up from the stroke, she wasn't taking her seriously).
Then a few minutes later, Miranda realizes the front door is open and Mary is gone, so Miranda runs out and is searching the streets of Brooklyn for Mary. Before long, she finds her, eating pizza from the garbage. It's obvious Mary doesn't see the problem with one nonchalantly eating from the garbage.
The scene then cuts to Miranda giving Mary a bath, in a very loving way, and Magda is smiling a bittersweet smile, watching this unfold. She basically tells Miranda, in her own Magda words ("You love.") that it was a very kind and loving gesture. To that, Miranda says "Don't tell Steve. He'd just be upset". She just doesn't want Steve to know his mother is that "out of it" that she would get lost in the city and eat garbage.
Hope this helps!