Based on this post. So far, mugumogo has always referred to herself and Maru as "housemates." I'm thinking maybe it's just an American thing for pet owners to refer to themselves as "mom" or their pets as "brothers/sisters." Even the post introducing Hana, last year, has a photo captioned with Maru saying "We have a new housemate." So I thought it was really cute that mugumogo is now referring to Maru as Hana's brother. :)