Why do people get upset not standing in the wedding?

knottiee7c54f205954336b

My fiancé's SIL informed her husband she is upset I did not ask her to stand in the wedding. She's upset that I asked his other SIL to stand. Me and her never spoke, but me and the other one communicate all the time. My fiancé is 1 of 5 brothers and all are married. I asked one SIL out of 5 to stand, the others and me do not speak. We don't speak due to they choose not to, one is a hermit, one thinks everyone is after her husband, the one I am discussing looks down at us like we are crap..... Then there is the one that is standing. His brother is standing ( or was.) messaged him to inform him of how she felt and then sent immediately " We see where we stand with your family." 
Why does everyone feel the need that they have to "stand" and if they don't its like the relationship is gone. Their children are in our wedding. I told fiancé if she wants to stand its whatever, but to inform his brother that she is to attend every event since its such a big deal for her to stand. 

Am I wrong? We have been together almost 3 years and every interaction specifically with her she said "hi" and that was it. 

banana468

I think weddings have a way of getting people bent out of shape.   

Can you ask her to do something of honor like say a reading?

If not, I think you can be clear as you were, "I asked other FSIL to be a BM because we have a friendship that exists in addition to her relationship in the family.  Asking her was not an intent to insult you." 

But know that your other FSIL and FBIL are the reason that kindergarten classes pull "invite everyone in the class or no one" for parties.   They didn't get that this was supposed to stop once they became grown ups.