I'd like to preface this by saying my FI and I are not evil people. We're actually very nice.
He and his mother are not close. She has a reputation as being childish and selfish. Her own sisters can't stand her, and her five children are still recovering from their abusive upbringing. If we could get away with it, we would not even invite her to the wedding.
That said, my FI was raised primarily by his aunt, and he considers her WAY more motherly than his own. His aunt and I get along fabulously, and we would love to have her help with the wedding. Unfortunately, his mother will become extremely jealous and start all sorts of family drama. Guaranteed.
What can we ask her to help with? We don't necessarily want her to feel excluded, but we don't want her to ruin anything.