I've not RTFT yet, because this jumped out at me:-
Of course he can't look after her at his own wedding! If our baby was here by then, Mil would be looking after it too! Surely that's what happens at weddings!?
You're quite right, OP. Of course your DP can't look after his firstborn child at his wedding to you... and of course, if your baby were here for it, you wouldn't be expected to care for it on your special day where you expect to be the focal point of absolutely everything. Be honest with yourself... do you want DP's firstborn there just to look cute in a little flower girl outfit as you play "happy families" for the benefit of the extended families - both yours and his? To perhaps make them all look at you in awe and wonder... not only at your radiant beauty in your wedding dress (every bride is beautiful, after all), but also at how generous you have been in allowing your DP's firstborn child to attend your day? To display, like a preening cock, your graciousness in smiling beatifically at her as she throws a tantrum, perhaps, or refuses to posture and smile to order for the photographs... because, after all, you are so much better than your DP's ex and the mother of his firstborn child, aren't you?! You won such a prize in him, after all, that of course everyone must understand your desire to flaunt his... not yours, but his... daughter at your wedding.
As a PP said, you sound very young. Immature. Perhaps this might not have crossed your mind, but maybe... just maybe... your DP's ex and this tiny child's mother, doesn't object to anything concerning you whatsoever. In fact, perhaps she is only being a decent mother by standing up for what is best for her child. Yes, your DP's firstborn has the right to have/build a relationship with her father... but that's her right, not your DP's and certainly not yours.
Actually, I have to confess to a little bit of bias, here. If this was 7 years ago, and your DP's child was (a) a boy and (b) 4 years old as opposed to 2, I might suspect that you were my son's putative stepmother having a whinge about the fact that I wouldn't allow my ex to use our son as a prop for his wedding. I like my ex's wife. I have a lot of respect for her, and I know that she loves my son as though he were her own... whilst understanding that he's actually my son. I spend time with my son's half-siblings (actually, I have them this weekend, whilst my ex takes his Mrs away as a birthday surprise... something which, incidentally, he never did for me!). But I didn't trust her, or like her one little bit when she first appeared on the scene. I was, actually, very anti her and objected strongly to her spending any time whatsoever with my son (who spends 9 hours EOW with his DF and his stepmother... their choice, not my son's, but that's not the point) because she did what you are doing.
She tried to be my son's mother.
He was only 2 years older than your DP's child is now, and was horribly confused by the wedding. But being 2 years older, he could vocalise that confusion... your DP's daughter won't be able to. But she will be confused and probably very upset by the whole thing... yet unable to vocalise that.
Your DP's ex is trying to protect her child from being hurt.
I suggest you take a gigantic leap backwards and allow her to get on with doing something that both you and your DP seem completely unable to do, yourselves... putting that little girl first.
And I sincerely hope that one day, you'll be able to build a relationship with the mother of your DP's firstborn child which is based on mutual respect and the desire to put his children first. Although I do have a sinking feeling that you'll also have to do this with the mother, perhaps even mothers, of the children he'll help create after you.
What a prize you feel the need to flaunt.