Routers are designed to pick up signals inside flats/houses, but not between flats and houses - the thick brick (whatever) walls will definitely weaken the signal.
Next door? Well, you could position her router next to an open window, and your equipment near your own open window. That would get a stronger signal, but there are security implications!
Ideally, of course, would be for you to sign up with an internet provider, who would come round and give you your very own router. Is it just a question of cost?
BB