How? Why?

I do not understand how or why someone would do something like this ...

My younger brother-in-law's girlfriend had been living with my MIL for the last several months sincer her step-father kicked her out of the house after her mother died.

She has been treated like a member of the family.

We have all done our best to make her feel welcome.

We are not a rich family and did ask her to find a job and help support herself, but never treated her in an unkind manner.

She was pregnant and did not tell anyone. Not until today when she went into labor.

She did not tell anyone she missed her period. She did not see a doctor about any pre-natal care. She never indicated that anything was wrong. She had gained weight, but she had been out of work for several months and had not done much during that period of time, so it was not odd.

She now has a very pissed off family to deal with. Medical bills (and since she did not have any pre-natal care, additional testing, and additional expense), baby stuff and clothing to be purchased, and a car seat to be purchased before Wednesday.

A boyfriend who feels betrayed because she did not tell him.

A family who feel betrayed because she did not tell us.

She has lived with us as a family long enough to know that we would not throw her out because she got pregnant.

I am at my wits end because I can't understand why she did this. Now, as a result of her duplicity, there is a great deal of doubt and mistrust regarding her motives for doing this.

I realize I sound like I am dumping on her, and right now I am. I will probably be more forgiving tomorrow, but tonight I just cant understand how she could do this to us.