This was long ago now but thank you everyone for your help. At what felt like the last minute one of my siblings donated to a kidney to my husband and they are both -finally- doing great!
I am grateful that it worked out for us that KS to the generosity of my brother and the grace of God but I am frustrated that the US did not approve travel visas for 3 weeks so that my husband's siblings could donate to him (an American citizen) to save his life despite being granted the emergency visa interview with the medical documentation we submitted, and each having jobs, children and a stable international travel history. Meanwhile, we know of others who have walked across the border with no permission and receive apartment/schooling/hospital benefits/food etc.
It makes one lose what little faith one might have had in the immigration system. I know that visas are not guaranteed but you would hope that for a medical emergency they would be approved. I thought we were done messing with visas and permissions after citizenship.
(For my husband we are but needed to try to figure out which things we could do to help his siblings be present to donate)
I hope anyone who sees this seriously considers becoming an organ donor!