Before my m-i-l moved in with us I called her Momma, and kept that up the first few weeks after she moved in. But soon it became easier all around to call her Bubby. For one thing, it taught the children to call her that; it was more comfortable for me; and as her memory diminished, it is less confusing for her to deal with me, whom she often cannot recognize as a family member, calling her by that title than by “Momma.”
My husband, though, won my parents’ instantaneous approval and respect by walking into their home after we got engaged and addressing them immediately as “Mom” and “Pop.”