NTJ. You used male pronouns to refer to your partner. As a man, he has NO IDEA what it's like to be a woman. We have to be constantly on our guard, even subconsciously, anytime we're around other people. Men just don't get it!
NTJ. In the USA, an "early teens" can't legally sign a binding document. So even if you'd actually signed a contract stating that you'd care for your siblings for the rest of your life, it wouldn't hold up in court. A verbal promise made by someone at age 13 or 14 is completely meaningless! There's a reason we don't let young teens make major decisions for themselves yet - it's because they don't have the life experience yet to make fully-informed decisions. What may sound "fine" to a 13yo could very well be horrifying to an 18yo who now realizes the full extent of what they agreed to do!
YTJ - that is "adult" information that should NOT be given to a 15yo! Yes, I know 15 is almost an adult, but the "almost" is the sticking point. OP especially did NOT have the right to tell Ella that kind of adult information when she's still a minor. My 17yo still doesn't know the full reasons why I divorced his dad, & they won't hear it from me until they're 18! Before their 18th birthday, info such as that described in this post could be considered "parental alienation," & the fact that a stepmother shared the info doesn't make it any less alienating. At 15, you can say something like, "No, it's absolutely not because of you! It's because of some issues between your dad & me. I know it's frustrating when you're so close to being an adult & really want answers, but I need you to just wait until you're 18, & then I'll be more than willing to answer all of your questions! Here's how to contact me when you're 18" & then give her a phone number or email address that you won't change for another 3+ years. (Assuming that as an ex-stepmother, there wouldn't be regular contact like there would likely be when bio parents divorce.)