Working and house training a Lab?

Hey cool! So will you get to have your dog at the day care for free since your sister works there?
 
I think if you sister is there to moniter, that's a great idea!
 
Just be sure the dogs fully vaccinated

It's not really the most desirable scenario to work FT and get a puppy, especially if you have a young child too, but it is do-able.  It just takes a bit more thought and effort is all and I think you've pretty much covered everything....  I think a walker coming round twice a day to see to the puppy while she has so little control over her bodily functions is a must. 
 
I would advise against putting pee pads down that attract her to use them for a toilet.  If you want her to use outside, then get her out there right away - it's less confusing and she is likely to be cleaner much quicker.  By all means put down paper, but PURELY to make the area easier to clean if she DOES have an accident, don't encourage her to use them.  Make sure she gets the yummiest treat you can think of for "going" outside, reserve this for toilet time ONLY.  Leave a pot of them by the door for the walker too, so she learns its just as worthwhile waiting for the walker to let her out as it is waiting for Mum.  I use something like hot dog ssg, cut up small - smelly, moist and palatable and pretty much worth killing or dying for, if you're a dog.