I've dealt with a family like this on a smaller scale

The money demands and pressure on my partner from their family was constant and outrageous (just much smaller sums)

They literally believed that if someone else had money and they needed some, you should just give it over

They had trained my partner to always give in which left us (me) unable to pay our own bills on occasions

The only solution I found was simple but ....

"There will be no more money going to your family or there will be no more marriage if you do"

I was hated by the family which was nice, as it meant I low contacted the shit out of them

My partner knew handing over cash ended our marriage, that also ended her heavily subsided living so she fell in line quickly

The only downside was I found that partner had the same mentality that other with money = fair game to leech off

You'll have to be firm here or your partner will break as they have had fron childhood to now to train her to think this normal behaviour