Officially a stepmom, though I don't exert myself as a parent to my stepkids. Rather I put myself in the role of another adult who cares about them.
I've found that family occasions, especially weddings, that bring together the original family members are fraught with tension and family politics. Sometimes, it's hard for everyone to not be ultra sensitive and reactive to perceived slights, even if the other party has the best of intentions and tries to be as accommodating as possible. From the steprelatives' point-of-view, unless there are a large number of them, they've been invited to a get together where they are the odd ones out. Officially, they are family, yet they're not because they haven't established the same emotional ties that come from having a shared history or shared blood. It can be overwhelming. Which is probably why your stepmom might come across as a little antisocial - there are three of you (sisters) + partners + your mom, whereas, it's just her and her daughter + your dad.
Based on your post, you and your sisters appear to be as inclusive as you can be, given that your stepmom didn't raise you and only became a permanent part of your family recently. However, I don't know your stepmom and stepsister's side of the story. There might have been things that were done or said that were indeed unreasonable. Even using certain words can seem presumptious and divisive - it might seem as though you have all the power and she has none.
That said, I can understand how the speech made by your sister's fiance's dad was hurtful. From your stepmom's point of view, she and your dad are together now, so mentioning your mom and your dad in the same sentence makes her feel like an afterthought and excluded. However, given the context of the speech, including her in the thanks wouldn't have made sense since she had nothing to do with giving birth to your sister or raising her.
As for what to do, if you have the time/energy/motivation, continue to find room in your heart and life for your stepmom and stepsister. Try to be patient and gracious to the fact that your stepmom and dad are trying to establish their own family - as far as your stepmom might be concerned you, your mom and sisters had your chance at family unity with your dad and now it's her turn. Therefore, she might come across as controlling when really she's just trying to establish some norms for her new family during a very vulnerable time.
Continue to extend occasional invitations to your dad, stepmom and stepsister but be prepared that only your dad might make it or sometimes he might not. Try to support your dad and respect his decisions. He's stuck in the middle and he's trying to do his best to please all the people he loves. Also, bear in mind that he is a grown man and it's not fair to blame your stepmom for his decision-making.
Finally, reaching an understanding can take time for blended families. However, establish boundaries for yourself. If your stepmom and/or stepsister continue to be abusive, revert to disengaged politeness. Don't allow any toxic behaviour to become a permanent part of your life.