My mom is trying to pick a groomsmen...?

Ok, well my bridal party is 6 girls and my fiance had 6 men that were picked. Well, some cirstances came up and one cannot make it up for the wedding. My mom feels like my stepbrother(who we have known for 2 years) should take this guys place. My fiance' said he has more close friends and if they can't do it then he will just make it 5 groomsmen. Well, my mother called me and got VERY ANGRY with me...saying "i want to know what his problem is with trent"..and i said "nothing he said he just has some other close friends" and she starting yelling at me telling me she is going to cancel the wedding and we should elope..i hung up on her saying "this isn't my fault". I don't know what to do??