I really feel for you op. I agree with pp that your SDs behaviour is utterly disgraceful and you shouldn't stand for it.
But, I'm also with @Milkshake7489 on this one. I'm a step daughter, twice over, a fortunate one with loving step parents. I also don't have half siblings to compete with.
However, I know exactly how I'd feel as a teenager if I did and how I'd feel if I found out that they had had more saved for them by parents.
Raging insecurity and jealousy would probably just about cover it - as an adult its very difficult to regulate those emotions, even when you know you're in the wrong. For a 17yo with hormones, trying to find their place in the world and with a brain still not yet developed to adulthood? Nightmare.
I was wondering how integrated your role as SM is and you seem to have answered it by saying that SD1 calls you mum and you love them both the same as you do your own children.
This is the point. If so, then why has the playing field not been levelled?
I get it, I really do and I definitely don't think you're an evil SM in any way. But this isn't just a childhood/adolescent issue. How many rifts occur because of Wills?
I'm not saying you should have put your own money away for your SDs. Im just pointing out the discrepancy between what the relationship is perceived to be and what it is perceived to be in reality.
Your SDs behaviour is completely out of line, no doubt about it, but im going to go into teenager behaviour because we all know how awful it can be.
I would think your SD2 is also looking to you to be the mother figure she doesn't have in her mum and it must be terribly shocking for her to feel that she doesn't have it in you too.
Nor am i saying that you and your DH should give them the financial difference. But I do think you need to discuss with your DH how to resolve this situation and how finances going forward are going to be split between your DDs and your SDs. Like it or not, whatever you decide is going be seen as reflective of you think about your SDs.
If you don't feel the same about them as you do about your DDs, that's ok - it really is. But you need to find a way to address the insecurity and the envy became if you don't, it will fester. Some might think its easier to be honest with SC rather than have them believe they sense a lie.
For what its worth, I think you sound like a lovely DM and SM. It's just, this is massive thing for a teenager to get their head around. 