Well, I sort of overall disagree with few comments.
I'm passionate about the horrible situation of pet overpopulation, so anytime I see someone willing to open their home to a dog I feel happy.
We have four dogs, each of whom were rescued as adult/grown pets from animal shelters. We also have three young dds. Two of our dogs we adopted prior to starting a family, and the other two were adopted after our dds were born.
Each of our dogs are absolute saints with our children.
And I hate the myth that dogs who are in shelters have issues----most of the time it's the people who owned them who had issues...most people treat pets like disposable toys.
There are millions of amazing and loving dogs awaiting homes in shelters/rescue groups!!
I would highly suggest your sister work directly with a rescue group since she has a young child. These groups will be very familiar with the personality and specifics of dogs. They're dedicated to making a good match with families for their dogs.
Also, very importantly she should expect to get a grown adult dog!!!
And shedding is an issue to be considered. None of our four dogs shed, thank goodness! It can be a real pain to deal with, but not necessarily a deal breaker if she falls in love with one who does happen to shed.
Whether she wants a certain breed or a mixed breed (my favorite!) there are a bazillion dogs out there who desperately need homes.....with the right personality match it can be a wonderful thing for both your sister, her child and the dog. Good luck to her!!