Jim affects Laura's internal conflict in that he is the only support system whom she trusts blindly, and yet she can see clearly that he is falling apart as badly as the rest of the family is. When Jim returns drunk from the movie theatre he basically spells out his feelings towards their current situation.
It must have been very scary for a character with a personality like Laura's to see the head of the household, the savior of the home, and the only man whom she can trust, her brother, lose his marbles and become slowly an alcoholic. She is aware of her mother's incapacitation as she is less and less reliable and more and more given to the past.
In conclusion, watching her brother fall apart is definitely adding to Laura's already traumatized existence.