This mother's blues and hostility sound like a combination of things to me. 

First:  a mixture of post-partum reorganization with tinges of post partum depression.  Especially with other children at home, mom has little time for the taking in and self-nurturing phases that are part of her postpartum recovery.  She has to jump full scale back into taking care of others besides herself and her new baby.

Second:  Studies have shown that increased prolactin creates an increased
protectiveness of the baby, and in aggressiveness in protecting that baby.
Rat moms will staunchly protect tissue paper in their cages, against any approaches by outsiders.  Transfer this to human action, and you get a mom fiercely protective of her new infant.  

Both of these combined, give you a mom very aggressively defending her space with her new infant and her time for "mourning" the end of the pregnancy.  When mom says it's probably hormonal, she's probably right.  But that of course doesn't make it necessarily better.  I see many lactnetters have anecdotes to share, which may help her feel more comfortable with her own responses.

So here is an anecdote to share.  My first born (and only child of my body) was born at 29-1/2 weeks gestation, almost 25 years ago.  Needless to say, he was in an NICU.  Even then they were very advanced, had me doing much handling and cuddling of my infant, even though he was not breast fed until he was 10 days old.  In the meantime, I was pumping my breasts q 2 hours to ensure plentiful milk -- and there was indeed plentiful milk.  Which suggests that I had very high prolactin levels as well.  I would sit in the rocker holding my premie (even before we started BF), with this picture of us in my mind --
me a momma human, with my child gripped in my teeth by the nape of his neck, sneaking down the street, outside of the hospital, slinking behind trees, telephone posts and garbage cans, to get to my lair (8 blocks away) where I could curl up in bed with my infant and growl at all who dared to come near.

In the meantime, I had a hard time not snarling out loud or lashing out at others (drs, nurses, etc.) handling my child (since they had me doing everything for the baby when I was there, including tube feedings).  I had no other children, but I did find myself snarling even at my husband for a few weeks, when he had demands while I was breast-feeding or co-sleeping.

Chanita, San Francisco