Relationships My older sister recently got married to her long-time fiance. They are polyamorous and both have a separate partner each. I have met my sisters boyfriend a few times at holiday events and he was really nice.
My brother-in-law's girlfriend (we'll call her Sally) I've met once before at their engagement party and she seemed nice but she also seemed like the type of person who wants to be the center of attention.
She talked over everyone, insisted on helping my sister open any gifts they received and she even told people that "she's the reason" my sister and my BIL were getting married because she's officiating the wedding.
It all rubbed me the wrong way, but as long as my sister and BIL were cool with it, who was I to say anything?
Things did get a little weird towards the end of the party, however, because Sally got pretty drunk and started very loudly proclaiming how monogamy was ruining relationships and was disgusting.
That the only reason people aren't open with their love is because they are scared, insecure, jealous, and controlling.
This made most of the people left at the party visably uncomfortable and most left soon after because she wouldn't stop (even after my BIL took her aside and told her to calm down).
I was helping my sister clean up from the party when Sally started questioning me about my stance (my boyfriend had to work so he wasnt at the party). My sister tried to squash the discussion but Sally ignored her and asked again.
I very calmy explained that I tried polyamory once and quickly realized it wasn't for me and that I was now in a very happy monogamous relationship but totally support non-monogamous relationships.
Sally started to say something but my sister very firmly told her that she was drunk and to go lay down. She rolled her eyes but did stomp back to the bedroom.
My sister then explained how Sally was usually very nice but did make being poly her whole personality, which had caused problems before. Flash forward a few weeks later to the wedding itself: everything went great! It was beautiful and everyone seemed happy.
During the reception my boyfriend and I were talking to my mom and Sally approached us to say hi (shes met my mom quite a few times and my mom is the type to love everyone). My mom reintroduced me and also introduced my boyfriend.
Nothing seemed off, we all said our pleasantries and that was it. A little later, my boyfriend went to get some drinks for us while I danced with my sister. It was taking him longer than I expected, so I went to go looking for him.
To my suprise, he was being cornered by Sally near the drink table. His back was literally to the wall and everytime he took a step away from her, she would step closer. She was also rubbing his arm in a flirty way.
I couldn't hear what she was saying to him, but the relief on his face when he saw me told me everything I needed to know. The only thing I did hear was her whining, "oh, come on."
I walked up and grabbed my drink from his hand, which finally made her take a few steps back.
I asked what they were talking about and she very bluntly and flirtatiously replied, "I was just telling him how much I love gingers." (My boyfriend, obviously, is a ginger).
I simply told her, as calmly as I could, to stop being a fucking cunt and there were plenty of single people at the wedding she could hit on. She said something back but I was already pulling my boyfriend away and didn't hear nor care.
The rest of the night was uneventful other than normal wedding fun. A few days later, my BIL texted me to say I needed to apologize to Sally because flirting with people is part of her personality, and she didn't do anything wrong.
I told him that one day she's going to come across someone less nice than me and she's going to get her shit rocked. My BIL thinks I'm being petty and an asshole, my sister and mom think I was justified and that Sally was being disrespectful.