Have RTFT and have no idea why you've been getting such a hard time from some on here @Candlemascandy .
Can completely understand your hurt and bewilderment, and have recently realised that my SM of 40 years has never really liked me, unless I was useful to her, and has masked her antipathy until age related mental health exacerbation has made it crystal clear.
My own late DM, to whom I was extremely close, did on occasion make me feel somewhat irrelevant and excluded on occasion, because of her generational stoic and pragmatic approach to certain things.
While we can absolutely be "the grown ups", "respect their wishes" and make sure we're not "making it all about me", it doesn't mean it doesn't hurt, after all, we are still their children and we have those sensibilities ingrained within us. Any perceived rejection by a parent can be extremely hurtful, even if we can understand it on a logical level, and in your case, a step child is getting essentially preferential treatment which us quite savage.
So I can only commiserate really, as it appears nothing can be changed.
I also thing it's particularly egregious to have included you in a year's worth of planning only to change the goal posts at the end. They could have just "got it done" with no input at all if it is really such a clinical formality.
Families are complicated creatures especially in blended cases. On occasion I describe my lot as "putting the fun in dysfunctional".
Take care OP. More people "get it" than don't x