There was a story iu the NY Posy earlier this month..abouut their NY HEROES..one was a NYPD sgt..she donated a kidney to a perfect stranger she'd read about in her church bulletin..the blood type of these kids must be very rare..normally a match for kidneys isn't that difficult..
Another option, The mother could donate 2 of hers and get a transplant if the boys had bad matches. Not the best, but as a parent I would go that route....if needed.
It's not unusual at all for people needing transplants to be very difficult matches. Polycistic types, as I understand, are often difficult. And I believe there is sometimes some complication among black people, too.