Mary is never unfriendly to Fanny or intentionally cruel to her, but initially does not pay her much attention, instead spending her time with her brother Henry and Fanny's cousins (Tom, Edmund, Maria and Julia).
When Henry, Tom, Maria, and Julia all leave the area, Mary begins to seek Fanny's company on a regular basis essentially out of boredom, inviting Fanny to come hear Mary play on the harp and take walks together.