I just saw this on television, so I think I can fill in a couple of details.
After the disastrous trip to the department store, the father was angry with the mother; Rose put Maggie in another room with the music turned up loud enough so that Maggie couldn't hear the argument between their parents. At that point, Rose must have been old enough and smart enough to realize that her mother was impaired.
Previously (it had been revealed in a conversation between Rose and her father) the mother had gone mostly without psychotropic medication because she disliked the "fogginess" that it produced, and the father went along with this so as to please her. However, in that argument, it appeared as though the father finally was threatening to commit the mother. Two days later, the mother killed herself in a traffic accident; the grandmother got a letter a couple of days later saying only, "Take care of my girls." However, she did not because her son-in-law prevented her from doing so.
It does seem as though the mother was bipolar. No normal adult would stay up all night making a huge amount of fudge and then take it in first thing in the morning to a major department store and expect to be immediately given a booth to sell the fudge. However, that is exactly the kind of thing that a bipolar person at the height of a manic episode would do; the illness gives one enormous energy and self-confidence, to an absurd degree.
As to why the mother killed herself, either she had crested the mania and was beginning the descent into depression, or she couldn't face the possibility of commitment (and, obviously, being medicated as part of it) or both.
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