R.,
What I am most concerned about after reading your note is that you have been married for 21 years and haven't worked thru these family issues with your husband. The real problem has nothing to do with not being invited to your step-daughter's wedding, it has to do with ignoring the reality of a broken marriage. Clearly, there's no communication in your home or you wouldn't be in this dilemna. Your husband not seeing his child for 24 years is so sad and then visiting his child without you is also another disconcerting indication.
You don't speak at all to what your relationship is with your step-daughter. So, if you haven't built a trusting, loving relationship with this child, why would she invite you to her wedding?
So, please stop blaming his daughter and his ex-wife about the dilemna you find yourself in. You are not a victim, you are just acting like one.
Ok, so without knowing the REAL details of what happened 21 years ago, you need to sit down with your husband and decide what relationship YOU are going to have with step-daughter and you BOTH need to put a plan together to help evolve it and then pray that it happens with grace and forgiveness. Unfortunately you are 'behind the eightball' because a major family event is going to happen before you can do this and that is truly the tragedy here.
So, you need to give her 'grace' beyond what you think is 'fair' and put her first, because it doesn't sound like you both have often done that for 24 years. It doesn't sound like she even knows you, so you should write her a thoughtful, heartfelt and GENUINE letter congratulating her on her special day and how much you hope it lasts lifetime. Do not send her a separate gift but rather make sure you and your husband send one gift from the 'family'. Take the higher road and behave as a 41-yr old, mature and responsible mother would, not as a teenager that has gotten their feelings hurt. It's her day, not yours. You had yours 21 years ago when you married her father. A father does not 'lose' contact with his child, he chooses to not communicate for 24 years and rationalizes his behavior.
Treat her as you would like to be treated or at least treat her as you would your own child that has been deeply hurt.
I wish you good luck on being a positive role model and parent.