I don't really understand the first part of the question Because the artist made her look similar?
And she calls her Mama in the same way that Dan does. She's part of the family, so I imagine it became natural at some point.
So it's completely unrelated but I just had the compulsion to share. I read kris's comment from bottom to top, right to left to right (read the second sentence then read the first then reread the second), as if it was the most natural thing in the world.
@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
@aiyikes my daughters cat is still fucking alive... the bastard is 19 and still kicking it like a kitten. I swear I think the thing is her familiar from hell. Seriously though.. he's an awesome cat.