Thank you!!
Ladies, thank you all for the valuable advice!!
An update on my situation - any advice welcome.

Sorry, this is very, very long

My parents had a live-in housekeeper for 18 years; she was with my family since I was in grammar school. After my mother passed away she moved on, and is now working for another family in another city. She spent the last couple of years with my parents basically taking care of my mother during her illness so my father could work. You know, too many memories, etc. to stay on at my dad's house after my mother's death, so she has moved on but stays in touch occasionally.
Long story short:
Former housekeeper is not a citizen but a resident alien, and has a work permit authorizing her to work in the U.S. - but only for the family she currently works for; if she wants to go to another job, she has to change her paperwork, etc. So, she has suggested her 23 year old cousin, who is looking for a live-in domestic job. The area we live in is desirable to this girl, but I just wanted to list some of my concerns to see what you all think about them - am I being overly cautious, not cautious enough, etc.?
The pros as I see them:
My dad knows the cousin, and if our former housekeeper recommends her, we know we can take her word for it because our former housekeeper is extremely dedicated and trustworthy - she would never recommend anyone (even though it's her cousin) she didn't feel was good for the job. She knows and loves my son, and would never recommend anyone she didn't think would be a good fit for my son and our newborn due 8/25.
What we would pay her would be less than 1/2 the cost of day care for 2 kids - this would be an enormous savings in terms of time and money, i.e. paying day care, carting two small children to school each day, picking up, etc.
Having her live-in would include her taking care of the kids during the day, cooking dinner, general housekeeping and laundry. For purposes of relief, I would not ask that she watch the children after DH and/or I return from work, on weekends, get up with them at night, etc. except under special circumstances.
The cons as I see them:
This would mean having her move into our house, and that would be a huge adjustment for everyone - a lifestyle change. No more lounging around in undies, no night-time 'escapades' anywhere but behind closed doors.

We would need to set her up as an employee, i.e., pay the employer/social security taxes - this would just be a minor hassle in comparison to the other things, though.
My son loves his school and this would mean taking him out of there and losing the interaction with his friends, the structured environment and activities that have been so good for him, and the play facilities especially for kids. He would also no longer have the instruction of the pre-school teachers there who teach the kids a pre-school curriculum.
As far as I know, the girl does not know how to drive and doesn't have a driver's license. I would also really want her to take a first aid course, but I don't know how feasible that would be - finding a class taught in Spanish.
Although I speak Spanish, the girl does not speak English, and so would not be able to communicate with my husband.
I worry about having her care for a toddler and newborn day in and day out. What is it going to do to her? How will she handle it? What if it doesn't work out, then how am I going to gracefully let her go - I'll also be back at square one with regards to child care. Will she be able to successfully manage an infant and very active toddler without going bonkers? I guess that's a question that can be answered only by having her actually try this out. I'm sure this is true for anyone, but given the fact that she will have essentially no support system other than myself and Dh around, I don't know how she'll handle the isolation.
Am I just worrying too much about all this? Having this girl live-in would certainly save us a lot of money and would really help with the household duties I can't seem to find the time or energy to do or do well. But I definitely don't want to sacrifice what's best for my kids in the interest of money or my energy; I'd rather spend the money on taking the kids to this great pre-school my son attends rather than worry all day that the kids and the nanny are ok. *Sigh*
Hey, how come my OB never gave me the manual on how to manage your life after you have kids??

:
Follow Mothering