User blog comment:LastationLover5000/Fairy Tail 283 - In Which Nakama Is Spoken (Natsu vs. Sabertooth)/@comment-4728047-20120519164537/@comment-3061966-20120519164936

Everyone else calls him Master, thus logically she is likely his kid-- whether it's by blood or adoption is another story. Doesn't seem like a term of endearment like say the Whitebeard pirates with Whitebeard himself.