That is the pits...when my sister was married in the KH, I was not df'd, but I wasn't attending meeting...I was allowed to be her Bridesmaid....my brother was df'd at the time, he attended with his family and was treated as if he were a wordly person.
In this case your Mom isn't getting married at the KH (what's up with that??), so I'd go anyway for the wedding and see how things go!
Hugs and empathy!