You can't do this. They will not give your sister a visa to be your nanny. Au Pairs can be obtained from Europe, but they cost money. Your best hope is that they give your sister a visitor visa good for 6 months, but if she is young and single they probably won't. It is not even worth the application fee to try. But bring that baby! Make it work. alternate your class schedules, do what you can. Your family should be together. Good Luck.