I have 4 cats 2 female and 2 male cats all adult. over the last few months the neighbour across the road has ?

letting the cats in her home, she even feeds them,now they wont come to back here, they sit on her window ledge alot she opens the window to let them in, and the front door, even when i open my door she opens hers and they go in there! I am furious and unsure what to do! is this cled as stealing? because technically they are not here! she has her own cat and i miss mine!