I am blown away at the number of PPs suggesting that OP's mother attends the wedding and makes a point of gossiping with other guests about personal family drama and about how 'disappointed' and 'hurt' she is/others are in DN etc.
Much as I understand the desire for others to know 'your side', that is just not on. Not the time; not the place.
Sure, if a stranger asks OP's mother if DB1 is her only son, or if someone known to her asks where her other children are, then she's well within her rights to simply and flatly state, "They weren't invited, unfortunately.", then allow the enquirer to draw their own conclusions about the cringingly grandiose selfishness and disloyalty he is displaying (and most will).
...But if you cannot attend someone's wedding ceremony and celebration without
gossiping about and trashing the bride and groom to the other guests
... Then just don't go at all; that is just gauche as f*.
Moreover, it almost certainly wouldn't reflect well on your family: Anyone standing there - deer-in-headlights for being unwittingly dragged into the bride and groom's family drama for asking an innocent and superficial question - might readily infer that such indecorous, boundary-oblivious behaviours could well reflect the reason the rest of you weren't invited.