Hi there.
I'm sorry the vet couldn't give you better answers. I know the more answers we have, sometimes the easier it is, the less likely we are to blame ourselves.

Whatever you do, don't blame yourself, you were a good meowmie to them, and made sure they were happy kitties while they were here. And just know that they are looking down smiling on you and probably will drop in for a visit now and then to remind you they are always close by.