The brothers, who fell out when Noel, 46, quit their band Oasis following a backstage bust-up with his younger sibling in Paris in August 2009, have been ordered by their mother Peggy to attend the family nuptials next week in Ireland, reported Daily Star.
The ceremony is to be held at the K Club in Co Kildare, where there will be free alcohol on hand, and Peggy has warned the rockers they must be on their best behaviour.
"Noel and Liam refused to go at first. They still don't want to see each other. But Peggy has insisted that they are both there. A few family members are expecting it to be very awkward," a source said.
Liam, 41, and Noel have barely spoken since their bust-up other than a few text messages exchanged and an surprise meeting at a post-Olympics party in 2012.