Dear BIL,
The thanks I get for helping support your family for the past few years, is an email asking me to co-sign on a loan to fund the family's education? Could you at least acknowledge the fact that I've been sending DS (your DW) a check, monthly, and most recently bought a gift card at the local grocer. So you want to get a PhD?! You've had a scheme for things "working out" for as long as I've known you.

There will be no more blood out of this rock. For my DS, the monthly checks will continue, but I will never co-sign on a loan with you.