What I see here are three brothers who need absolutely to stand by the decision you have taken. I feel very awkward about this because in essence what I am saying is that you should defy your mother's authority.
But I think in this case nothing else can be suggested. You three need to stick together, and Pat and you in particular need to make it clear that for you guys this party is just NOT happening.
Can you and Pat take this matter up with, for example, your family doctor and get him or her to speak with your mother? You can talk to the doctor in confidence and that confidence has to be respected, but the doctor can tell your mother what he or she thinks on the basis of what you have said. Perhaps this would help.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)