You're making no sense. If she'd done her due diligence, she wouldn't be here would she? OP has described a consistent and identical approach to step-children from all of the extended family, and a DH who said he would treat her eldest as his own (and she says he genuinely still does on a day to day basis) but also describes wills that treat both girls differently. Wills that have been in place for years. At no point has she said that he misled her. What she describes is that this wedding has been the catalyst for various realisations. The youngest has only been around a few years, anyway. OP herself says in her latest post that she "assumed" things about inheritances. Why would you do that? Did you, before you married? I certainly didn't.
Again, it's a fact that the onus was on the OP to protect her eldest child from the slights and treatment she perceives her to be facing now and in the future. She hasn't done this. She has put that child into this position with her own choices. Every post from the OP has been to criticise the DH, the MIL, the BIL, the SIL (although to decreasing extents as the posts have gone on)....and not once to say "oh fuck, what have I done. I've messed up big time". If she had done, she would have reeived very, very different replies and I bet a tonne more constructive advice on how to move on.
My point is to say that the OP got her child into this situation and she has to get her out. It's a complete waste of time blaming her DH's family for the predicament she put her DD in, and casting about for ways to get her DH's family to change so that the predicament vanishes. If you are the sort to prefer to see victimhood in suffering the consequences of your own actions, sure you can see that as being kicked when you're down. If you'd rather be pragmatic and seek to actually fix the problem rather than spend time blaming everyone, it's not a question of kicking anyone at all - that simply isn't the frame of reference.
I suspect this probably aggrieves you even more, and that we will disagree on this. That's fine. This is an open forum where everyone is free to comment or not, within the site's guidelines.