Your poor DH, he shouldn’t be put in this situation by his DD at all, but have some empathy for him and imagine how difficult this is for him now feeling torn between his own DD and SD that he has been good enough to raise as his own, and then having you add some extra pressure with crying and more ultimatums.
You have called your ex out for being a deadbeat dad, so do you seriously expect your DH to turn his back on his own DD for you and yours?
As a father he should not be willing to lose his relationship with his DD, to walk your DD down the aisle IMO.
Furthermore, would you be willing to prioritise the feelings of your SD over those of your own DD? As that is what you are now asking him to do. What would you do in your DH’s shoes?
You’ve not shown any interest in the feelings of your own SD in your post, about why she feels this way and how it can be resolved, do you care about that at all? If you expect your DH to consider your DD’s feelings so highly you should be willing to do the same for his DD, your SD.
Sounds like your SD is insecure in her relationship with her father for whatever reason, and it sounds like you couldn’t care less about that. Yes, she is being horribly unreasonable and selfish, but I feel like we are missing a lot of backstory here about why she has an issue and her ultimatum to me is hinting at some deep emotional pain somewhere.
It’s still unreasonable of her, but as PP have said parents separating and blended families raise a lot of complicated feelings for the children involved, particularly the ones who may feel they are losing a parent that a different child is gaining.
DH needs to have open conversations with SD about her feelings, seeing what reassurance she needs and trying to find a way that she knows she is loved and irreplaceable so she feels more comfortable with sharing her dad so that hopefully she will feel happy to let go of this horrible ultimatum, and your DH can walk your DD down the aisle as planned without risking his relationship with his own biological child.
Meanwhile you could support and reassure your DD that she is loved by DH, but he’s just been dropped in the middle of a shitty, unfair and impossible situation.