It could mean that Ponyo & Sosuke have the same blood & Sosuke might be half human. Lisa is a loving, self-sufficient, responsible, wise (like Granmamare) and strong-willed woman. she tries to raise Sosuke as best as she can(like Granmamare), she even knows many seafaring skills such as morse code with signal lamps. & when Sōsuke told his mother about Ponyo's return, Lisa realize ponyo had some to do with Granmamare (Ponyo's Grandma). Like Ponyo, she probably know how she's feels to have the desire to become human & wanting to live in the surface world (& had the same origin); she accepts Ponyo as her own without question. (like Granmamare) She is an outstanding mother and a great woman (like Ponyo).