How about writing a letter to your FIL? Putting yourself in his shoes? He is feeling anticipatory grief at the loss of a very precious relationship, grief is also about loss not necessarily death. His heart is breaking and so he strikes out in anger at what he believes to be the cause and a frightening threat to his future happiness. Be the bigger person, apologise , show compassion. It will cost you nothing but a bit of hurt pride, yet will give him something to look at and think about when things are looking very bleak. Also make sure your MIL knows how to use Skype before you leave and make a regular arrangement for you and your family to contact your in laws in this way. Put in every effort you can to reassure them of your continued contact. Above all be kind. One day you may find your son and a beloved grandchild are being moved away from you by a future DIL for similar reasons. Imagine how that must feel. Grandparents love their grandchildren with a passion that almost matches that of the parents but the have no say, no rights. Please be kind.
To go through chemo therapy or choose not to?