Sadly, I tend to beleive this as "straight up". yes the Brother plans to go into the monkhood, and yes, they want to throw him a big fuck off party, and yes, they budgeted 50K to be divided between the family.
And yes, they expect the girlfriend of the farang to stump up half the money. A few points that cross my mind randomly.
The party wil not only be booze and food, but also in attendence will be other monks - and they get the normal donation - everyone who lives here knows the sort of things - buckets of foodstufffs, robes etc etc.
The family is probably "sincere" in thinking this will be good for the brother - who will most likely spend 2 weeks there before deciding its all too much like hard work and slinking back (seen that a few times). The fact that the family knows this -won't stop them from pushing the boat out....
The mothers/MILs (??) -if you don't give me, I will take from the loan shark is ooohhhhh so typical of upping the stakes.
While I kinda agree with the posters who go along the "fuck em" line - this does put a LOT of pressure on the girl friend - who has a whole life time of brain washing that "family comes first". Sadly that can be very deep rooted. Forcing her into a corner will put a lot of pressure on her and the relationship.
If I understand the current situation -she works, and stays rent free in OPs house? If so, she can provide some money. I would (and I expect howls of outrage) meet them half way. tell the g/f she must chip in 5K and I suggest the OP chips in 10K. And make it clear that the lot.
I suppose the problem is, that as I see it - Thai families are like this - they WILL beggar the whole family to provide a party for the waste of space son. Sensible? No. But they way it is - in many, many cases yes.
I personally have spent 10 year trying to find the fine line between making it clear that I am not the familys ATM, but that I am also not "stealing away the daughter". We try to help meet the family commitments, but NOT as a blank cheque and on a case by case basis. over time the missus understands this, and will screen the more opportunistic crap out, and we can discuss the difference between what is asked for - and what is needed.