Drama before invites are even printed...

HisGirlFriday13

It's a long story, but it involves the police, warrants for arrest, drug-dealing, bad decisions and would-be stage intervention and arrest.

hacked

I thought, and I may be wrong, but a "step" relationship ends if the parent's divorce.  If your father divorces your step-mother, she is no longer your step-mother, but your father's ex-wife.  I would start by not using the term "step" in your conversations.  It personalizes a relationship that does not exist.  I would refer to her as "so-and-so's daughter".   So, "Dad, I did not know that you and Mary were back together, but as I mentioned before, we are not inviting minors (as opposed to "children", which is more loosely defined).  So, since Mary's daughter is a minor, we had not planned on issuing her an invitation" 

I think that it is harder to see a woman married to your father as a stepmother.  If she was in your dad's life when you were a kid, it makes more sense, since she did some "mothering".  If you are an adult and your dad marries her, I can see that she does not play the role of a mother in your mind.  Same for a step sister, as opposed to a sister you grew up with.  You are placing a name on a person for a role they never filled.  Hard to get enthusiastic about it.

laylasaurus

I think all this needs to begin with a discussion with your dad.  Let him know that you haven't sent invitations, and that he doesn't get to dictate who you invite. Inform him that you and FI have many important people in your lives and only a limited number that you can feasibly invite. You want the people you hold closest to you there, and Stepmom and sister just don't fall into that category for you.  Also let him know that you aren't inviting kids.  Tell him, as the others said, that SM is welcome to come as his plus one.  

During this conversation you may find him to be understanding, in which case sticking to your guns will be fine, or he may completely put his foot down.  If the latter, then you need to decide which is more important to you- Not having sister there or maintaining a good relationship with your father.  In the end, that may be the only choice you have.  Is not having stepsister there worth possibly setting back the strides you've made with your dad?  

I don't think anyone will have a problem with her being there, she is old enough to be exempt from the "no kids" policy, especially as "family".  However, you are completely in your rights to not want her there.  You by no means are obligated to invite her as "family" since you clearly don't feel a familial bond to her.