Threesome..

Has any family member ever asked you to be a part of a threesome? My SO recently got into a physical fight with one of my cousins, my SO deleted my cousins number from my phone, took him of mine and his Facebook and hasn't let me hangout anywhere where he's at. For a while I didn't know what has happened up until today. Turns out my cousin asked my SO if he could join us in a threesome!! Of course my SO got furious and went at him, I didn't belive him at first because I never thought my cousin was like that. But my other cousins who were with them that day heard everything and they have msgs from my other cousin telling them my SO 'overreacted'! I feel so weird knowing this and more because there were other family members around who heard this. What should I do? Has anyone else been through something like this??