Barista's are "trained" to be pleasant and helpful to customers and you could remind her of her favorite uncle, father, etc. I would definitely get more information about her personally instead of relying on just body language and general impressions. She might feel more relaxed with you because she does not see you as hitting on her which, to a working woman, can be a blessed thing at times. If you enjoy the relationship now, such as it is, changing how she may perceive you could wreck it?

Do you know if she has a boyfriend or anything about her living situation? I really liked my younger male bartender and he turned out to be gay. I would definitely learn about her lifestyle, siblings, etc. in a general way; what she does for holidays and what her hobbies are, where she went to school, what her future plans are; before showing my hand/interest in her in an other than simple, regular/familiar bar patron way.