Hi OP,
What a horrible situation for you, your partner and your daughter to be put in.
Without any more context about your SD it's hard to really give advice here. One assumes there's years of jealousy and hurt sitting with his Daughter. What is their relationship like? Also why isn't your SD having this conversation herself with her Dad herself?
One would have hoped that Ex W could have talked some sense into her daughter and not been part of this manipulation. I agree with many posters that it sounds like there's years of resentment and hurt sitting with the SD but assuming she is an adult it is her responsibility to work through that herself, not to project that onto other people.
Surely your DH can walk both young women down the aisle? The woman getting married has had this man in her life for 22 years... it's not like they've known him for 5 minutes.
It's up to DH to make his bio DD feel loved, special, wanted and unique in her own right and to build that strong one-on-one bond with her. But in my opinion that doesn't mean the bio DD gets to dictate that her Dad doesn't walk your DD down the aisle. If he agrees to that then where does it stop "Her kids aren't allowed to call you Grandad or I'm never speaking to you again", "You're not allowed to spend Christmas Day at hers and not mine or I'm never speaking to you again"....
He has to make his own choice but I'd be encouraging him to make sure his DD feels secure in their father daughter relationship. If she truly did then she wouldn't have to make such threats.
I can imagine it feels really hurtful for you and your DD who have likely both given a lot to your DH and possibly his DD too.
Remember to breathe, think, pause, respond, don't react.... and don't divorce him over this...he will have been pushed into "guilt corner" by this and he needs your compassion more than ever xx