Iz, m'dear...
I hear the hurt under your words. The "what did I do to get treated like this"??
The answer would be nothing at all... and the solution, well - let's start with hugs first. It's not fair, plain and simple.
This situation sounds a lot like what happens on the playground, with a small group of 3 girls. I've seen this happen lots of times - even with myself and 2 D's. Someone ends up "the odd man out" and "not included"; feelings are hurt; sometimes the troika dissolves completely... and former friends become enemies. I don't know why, but 3 is the magic number for this kind of thing.
But when it's family - especially immediate family - there's one more aspect to the situation, I find. Note that this is just my personal way of dealing with and explaining the same kind of thing, OK? It may/may not work for you or even apply.
I expect others to "play by the rules of fairness" that I do. I expect that my close family would know me well enough to know what I need; want; in the way of relationship - and that they'd want it too or be able to compromise in a fair way. Just as I would mother someone else in a situation - I EXPECT the same back. That's the ideal, right? That's what we're taught that family should be like, right? Siblings standing up for and alongside us, supporting us, in our life trials and tribulations. Daughters and moms knit together into almost one unit. Blood is thicker than water, right?
I am coming to believe that this ideal is someone's wishful thinking; fiction; that it might've happened once upon a time... and someone else saw that singular event and decided hey - this would make a really good marketing theme; an image of how families are.... I can sell that! You know? And so we got the "Leave it to Beaver" picture of one happy, loving family... and just like girls grow up dreaming that their wedding day will be all perfect, Disney-princess, and hubby will be the perfect man and they'll live in a cute cottage with a white picket fence and have happy, perfect babies...
I've dubbed this: "white picket fence" syndrome... sarcastically, of course. Another way to name it would be "Expectations"... which will fall somewhere on a scale from none to unrealistically high. I think we all build our dreams and wants into the expectation, unconsciously, and when reality doesn't match that - there's that sense of disappointment; being cheated out of what "could", and maybe what we think "should" be...
and then, we find our own personal way of dealing with it. NC, is one way... punishing ourselves, another; blaming our self for not being "acceptable" in the relationship... and some struggle on & on & on & on trying to get a different result from the same expectation, refusing to accept the real conditions, the obvious in the other person/people... some of us are emotional contortionists and will attempt to become pretzel-fied ourselves, IF ONLY it helps get our expectation met. (I think I've been through all these.)
SIGH....
I've decided that it's the expectation that is the problem; the thorn in my side. And if I can give up expecting that my family thinks like I do about fairness... automatically loves me for who I am... and will look out for my best interests and care about what I care about...
... well, it still doesn't feel "great", but it doesn't hurt as much and I'm not as surprised the next time they prove to me, that this is who/what they are. It helps me create a reality-based "baseline" about the relationships... and sometimes, shows me a direction or path down which I can proceed with them - AS LONG AS - I don't let those old expectations creep back into thinking. The repeats - the history repeating itself - is usually due to my deep down hanging on to those old expectations, unconsciously (I don't know that we can help ourselves... or really learn to let this go; we all want this connection instinctively). What I can do, though, is reaffirm my baseline - what I've observed about bioNic mom or GS Bro, what I know without a doubt about them - and then, adjust my expectation accordingly. I can teach myself to remember - what I know beyond a shadow of a doubt.
SO: with your sister and D... it's just possible that your sister felt she "knew the best way" to help you recover and didn't think it was good for your D to see you like that; scary for her... and that as a consequence, they developed a relationship with each other. Even tho each of them has a relationship (or did) with you separately - their relationship with each other existed(s) sans Izzy. And Izzy expects them to see how this would be unfair to her... and to consider what Izzy wants... to know what that is... and they're not hearing this. Just like there is voicelessness... there is a kind of delusional denying deafness, too. It's worst in Ns, for sure - but we all do this to a lesser degree - when we're uncomfortable with some fact or piece of reality.
When we're conquering voicelessness... a lot of times we learn to tell others how we feel. That is even a huge, huge step forward... but with those who are deaf to us we have to go one step further... and that is to clearly and assertively tell them what we want. With Ns, not even that is any guarantee - my bro has literally laughed and said "I don't give rat's ass what you want". But with people who aren't all that N... this is quite helpful and one thing it helps for me, is to bring my unconscious expectations down to earth, within myself.
Anyway... like I said at the top of this long-winded explanation... this is how I explain this to myself and it helps. I get used to - over time - not expecting my mom to ever ask a single question about me and my life, so I just let her talk when she calls. It's who she is. I'm still struggling with dealing with my brother; he's so disconnected from reality I'm not sure anything will get through to him until his own self-interest is threatened. I'm hoping to show him that my self-interest is his, too... but I really don't expect him to get that; to understand it. So if this next attempt fails... well, it's back to the drawing board and starting to set up ways to protect myself from his denial, delusion, and disconnect.