Actually,
Amber has used the word "ashamed" at least once - in relation to Zii, of all people.
She's also worried about telling her parents - if we couldn't deduce that from the whole getting Ruby to lie to them for seven or eight years thing, of course. She may be open about her porn career among friends, but her attitude has a lot of ambiguity and discomfort to it.
Anyway, yes, I caught the "I'll try not to sound stupid" when it first appeared - but my interpretation of it is
slightly different. I think that Ruby has, at some times, made a point of reminding Amber that she's the smart one - possibly even going right back to when she was 12, before Amber left home, if she was already doing noticeably better at school than Amber had (in which case it was probably accidental), and certainly after the disaster when Ruby was 14 (when it surely became deliberate). She probably wanted to impress her cool big sister, back when that was still how she saw her (and when Amber was locking her out of her bedroom). And later, even aside from Ruby's contempt for Amber's porn career, well, Amber was the pretty, self-assured one - Ruby would have instinctively tended to remind her who was the smart one, and just have shown off a bit generally.
(Oh god. Been there, done that, in a much, much smaller way.)
Bonus points if each sister feels that their parents favour the other one, because those parents are so anxious to preserve "balance" that they keep telling Amber about Ruby's cleverness and Ruby about Amber's prettiness. But yeah, whatever the details, Amber is probably a little bit intimidated, and this relationship needs work.