Not a therapist here, just someone who once had pretty miserable self-esteem and is now way past that.
I think WhyNot and Khiadra made some good comments.
First, I think a big part of one’s self-esteem is rooted in feelings of competence. A sense of accomplishment helps one to feel that they can do things. WhyNot’s suggestions give a good feel for making the first baby steps; later steps might be to do things like throw a big party, offer to give a public talk - and make oneself do it. I suffered through a couple of horrendously bad talks before things got better.
That last part of the previous paragraph is sort of the “make yourself put your hand in the fire” approach. Not a good starting point, but something to think about down the line.
And Khiadra’s mom’s observation is one that every mouse needs to hear. They’re not all lookin’ at you, and deriding your appearance/demeanor, etc.
In fact, they’d be happy to have you listen to their tales.
What can you do, as her brother? I don’t really know much about your sister beyond what you’ve said here. You can point out any observations that might accumulate here, and you can applaud her when she does make a baby step or a big step. But hold those kudos for the things that are truly accomplishments. She’ll likely quickly realize what’s going on if you applaud her for things that are not really steps ahead.
Another thing I’ll throw in, based on my experience as one who was once clinically depressed, and experienced a concommittant low self-esteem, and later dealt with that with my long time SO. If you spend a significant period of time in that condition, you can become most comfortable with it, and be staggerringly unaware of how to deal with any state other than, once again, accepting failure. So, whether consciously or not, said sufferrers tend to set themselves up for failure. I don’t know how close you are to your sister, but that’s one thing you might watch out for.
And, of course, if you know a bit about her life, you might direct her towards the things you think she might bag as accomplishments.
My own sister was tortured through high school and her first dose of college. And we were not particularly close. My mother used to try and get me to help her make friends. Ackkkk!!! I had very little self-confidence myself, and I hated those sessions when my mother tried to make me tell her how to be popular (She’s older than me).
I don’t know what that was supposed to add, but I will note that she was Homecoming Queen at her university. The point there is to tell you that, while she and I were both very put-upon by what we felt were the demands of lives we couldn’t muster for, we both got over it. I’ve been through some rough times since the depression lifted, but I’ve never been depressed since.
Finally, and probably only a footnote, as I can’t really articulate the importance one might attach to this in the way of recapturing one’s ego; I found that as I got better, I became much more willing to to bestow small acts of kindness upon strangers. Someone struggling with a flat tire, or fighting the stamp machine at the otherwise closed Post Office - if you can make it all work for them, that’s an accomplishment. One that nobody but you (and the stranger) will ever know about. But, it adds into the credit column.
Anyway, I’ve said enough. I hope some’s coherent. Good luck to you, pal. And good luck to your sister.
(If you happen to have the bad luck to catch another shy person, you can either hang around and comiserate with them or insinuate yourself into another group and try again there.)