@jak - This is probably out of habit what with living there for 10 years and being effectively adopted by the family. Note that Julia's also called the father 'Papa' in the time-skip. As to why Mama has a different room than Papa? Who knows, who cares? It's not uncommon for partners to have their own separate rooms, and considering that Papa probably broke his back from moving the bed, it's more than likely that Mama didn't use her separate room to sleep in.
Yes, it's probably a LOT safer for her to say 'mother-in-law' and 'father-in-law', but then everyone would be confused and only Alex would give us the straight-man response. (cough - his famous dead stare). I mean, we already got enough of that when Julia has shown that she has plenty more booty than Mama... XD