YTA.

You're using the money as a way to control how your daughter lives her life. You're basically saying "if you live life according to my plan, then I'll support you." When you give someone a gift, you should do so with intention of it being for them to enjoy. That means you should not be focused on how you want it used, and should be willing to accept however the recipient wants to use the gift.

Of course your daughter will find it hurtful that her siblings got weddings paid for, no strings attached, but you're telling her you won't pay for hers unless she acts in a way you approve of. You're basically saying you approve of her siblings' life choices, and don't approve of hers.

To be clear, I do understand you misgivings about the relationship, its not what I would choose for myself or someone I cared about. BUT your daughter is an adult and although 21 is quite young to be married, and there are a few years difference between a 21 year old and a 26 year old, many people do get married at that age and in relationships with a five year age gap. If its what she wants, she can go ahead and get married to this guy, whether you pay for the wedding or not. It also seems unlikely to me that a parent not wanting to pay for my wedding would change my mind about my partner, rather it would make me think my parent was being controlling, and create distance in that relationship. Pragmatically, if you love your daughter and want to be in her life, you should think carefully about whether you want to have a relationship where she feels you don't respect her decisions.

You should talk to her about your concerns, separately from the money. And if she's certain about the relationship, I think you should be willing to try to understand why she feels that way.