Wedding guests

So, I'll try to make this as short as possible. My sister and I have always had a troubled relationship and somewhat recently we got into a spat and haven't been on speaking terms for ~6 mos. I called her about 5 months ago and told her that I wanted to work on things but she basically slammed the door on that one and we haven't talked since. She recently sent in her RSVP and said "+1", indicating she is bringing her BF. Firstly, my fiance and I are having 35 guests at our wedding in order to stay within our budget but more importantly so that we surround ourselves with people we are close with and who know us as a couple. My sister wants to bring her BF because she wouldn't be comfortable going otherwise. She knew we weren't allowing guests. Plus, neither I nor my fiance have even met her boyfriend....So, um, a wedding guest count of 35 (super small) and one person we haven't even met? I mailed my sister a letter asking if the four of us get together so that we could meet her BF and she declined that offer because we're not on speaking terms.

What should I do? I feel like either way, we are dammed. She's been really mean to me (not wanting to work anything out, not being there for me in the process of getting married, etc.) and I feel like why should I do her a favor? Also, we're not even allowed to meet him, yet we have to invite him to our wedding? How does that make sense? I feel like if we don't invite him, she won't come and the rest of the family will be pissed at us for not inviting him. Yet, if we do invite him, she still won't be on speaking terms with me and we'll get a total stranger at our intimate wedding.