I agree with Suz and B.
It's your expectations that are letting you down - not your MIL. She's just the type of person she is. Her personality is just that. Just accept her for who she is, and move on.
My in-laws want my husband and I to be people we're not, and this has caused them so much upset. My husband keeps saying "um sorry..." but they can't move on from it. They have said we're ruining their retirement. It's like well, no ... we're just who we are.
I just glanced back through your previous questions, and your husband's oldest daughter had huge issues. Maybe your MIL is like my mom. Not cuddly (somewhat emotionally unavailable) but very giving to kids/teens in need. My mom always made a point to spend time with her step-granddaughter who felt very insecure at family gatherings. She had a lot of issues due to her parents' divorce. My mom recognized that and made an extra effort.
Your daughters have both parents there and maybe she feels they are very loved and secure. Who knows? Maybe she senses that your Step daughter needs a tad more attention. In your previous question, you mention she had been cutting and had been going off to military college. Without reading it all - sounds like a troubled youth.
I wouldn't take it personally. I also wouldn't get involved (i.e. talking to her yourself).
Just let it go. She is who she is. I agree about getting out and getting coffee and talking to dogs - with B. Maybe that's just her. My mom used to get up and go for walks early in the morning while the morning chaos went on in our house. Maybe she's an introvert - who knows?
I get what you are saying. I am a sensitive person. Just take a step back and realize, it may have nothing to do with you or your kiddos. That's usually the case :) Don't waste any time on it.
*ETA - I think Marda makes a good point in that perhaps MIL feels judged. Sometimes people pick up on the fact that they feel you've disappointed them. Where she's been given a ticket as a 'hint' etc., she probably knows you and hubby are not happy with the kind of mother/grandmother she is. Who knows, maybe she has depression (withdrawn, doesn't want to travel etc.).