<human>: Condense the text into a few key points

I'm getting married in September. When we got engaged my fiance's sister was involved with a guy. We naturally set aside room for her to bring him. However, this relationship ended before we sent our the invites. (Still haven't yet.) We then took him off the guest list and brought someone from our standby list on. 

My fiance and her sister are close and often talk about the wedding. It came up that we no longer were expecting her to bring anyone. She got very upset and insisted that she be given that spot. She says that it will make her more comfortable to have someone there to share that special moment in the family.

We've said that if she starts seeing someone we would gladly be willing to give her back that spot, but not for *anyone*.

We have a limit on how many people we can bring as the reception area is small. My feelings are that it is ridiculous that she would expect us to have one of our friends crossed off the list so she can bring someone we have never met. To top it all off, she doesn't know who she will bring. Just that she wants to bring **someone**.
<bot>: My future sister in-law is insisting for a plus one even though she has no idea who she will bring.