Once again OP, you seem to be avoiding the question as to WHY your DH won't speak to his DD directly about this.
Why do you so firmly believe the story that the ex and you SS have told your DH? Is there more to this than you've actually posted, as by the nastiness displayed by your SS, I wouldn't be at all surprised?
You say that you don't think that your SD knows that her Mum & Brother have told her Dad this 'story'. So what if it isn't true? What if your step son is just stirring the pot, which I personally think is highly likely from his previous behaviour?
For goodness sake, before disappointing your own DD, at least get some clarity on the situation, even if you have to call your SD yourself, and ask her. It doesn't have to be nasty, you can just say you understand that she's unhappy at the prospect of her Dad walking your daughter down the aisle, and then don't say anything else until she responds. (If this isn't a tactic you're familiar with, it's well known that people can't bear silence in a conversation, so will 99% of the time, say something to fill it). If she says she doesn't want to talk about, then fair enough, go ahead and speak to your daughter about you walking her down the aisle, if that's how you want to resolve this, but please at least confirm that this isn't her dear brother shit stirring, as from what I've read, unless I've missed something, it doesn't seem like your SD, is the one who has been nasty about your DD at all.