Still self-sabotage despite self-awareness

I'm so disappointed in myself.

I self-sabotaged yet again, when asked the name of my law firm by my brother in law.

Some historical facts about my relationship with my BIL:

1) Until I got into trouble at his temp agency that he had recommended to his trainer to hire me: we were on great terms.
a) I wasn't getting any assignments and complained to the trainer who then fired me for complaining. After that my BIL lost all respect for me.
2) After what happened at his temp agency w/me (I never involved him either); I caught him once glaring at me and he avoided being in the same room with me for the past two years until recently when my sister would invite me over for her children's birthdays or a federal holiday.
3) I didn't tell my BIL that I know the name of his new temp agency where he does the IT work (I heard him in the background talking to a recruiter when I was there to apply as a temp this summer, which I never completed the application for once I heard my BIL's voice). I later verified his employment via his LinkedIn profile since we were still linked as connections there.

Since my mom died, my sister has continued to host federal holidays and I continue to attend, despite the stilted and fractured relationship we have as sisters; despite the fact that her husband doesn't like me, and her children who are now in their 20s, treat me with detached affection.

Last night at Christmas dinner, my BIL asked me the name of my law firm.

What I wanted to say:

"How about I tell you that when I've been there for a year."
"Where do you work?"

What I said:

"The name of the law firm."

Regret immediately sunk in. I suppose it wouldn't matter anyway, had I updated my LinkedIn profile with my law firm's name and my role there. I haven't for obvious reasons.

I feel disappointed in myself for not holding stronger interpersonal boundaries with my BIL. He can reach out to the firm administrator and warn her about me. He could do a number of things I guess, digitally, to sabotage my employment if I he wanted to since he's been IT recruiter for 25 years and knows everyone in the temp industry.

I've been trying to reflect on why I still let my sister and her family cross my boundaries. It can't be as simple as, me wanting them to like and respect me. It has to be something more. But, I can't put my finger on it.

They all have very strong boundaries with me because they have nothing invested in me, so they have nothing to lose. I never know anything about their lives unless they tell me. And even then, I just assume what they share with me are blatant lies anyway.

I'm so disappointed in myself for telling my BIL the name of my law firm. It shows me that I'm still people pleasing when I know the people are toxic to my development and well-being. Am I so well-trained in black sheep, self-loathing, codependency that the patterns of emotional abuse put upon me by my parents and siblings are etched permanently in my brain that no new synapses could overwrite?

Therapy (cognitive, DBT, CBT) has not been successful at overwriting those trained responses, put upon me against my better knowledge as a child. And although I'm very aware of my automatic responses, like a pre-written out of office email signature, I can't overwrite them. No matter how may therapists I talk to, how many books i read, how many journal entries I write. I can't overcome those damned automatic responses like was seen last night.

I feel like I'll never succeed in overcoming those automatic responses. I'll never be able to have a correct boundary with a toxic family member.