Thank you everyone.
Unfortunately, I have lost this fight and we have postponed our wedding. My fiance's family is not getting much information at all from my future father in law's doctor about what he is supposed to expect from the chemo and after the chemo. They have not been encouraged to seek out information or a support group or anything. They are very stubborn and set in their ways and nothing I nor my fiance suggests is being listened to. What this boils down to is that if the doctor doesn't tell them, it isn't true, and the doctor won't tell them ANYTHING. So they think that because he is feeling worse right now (he just had his 5th out of 6 chemo treatments), and the doctor hasn't told them otherwise, that there is no way he will feel up to attending a wedding in October. My fiance and I have tried and tried to explain that the opposite is likely to be true, especially since the tumors are shrinking, but we are not doctors and so they won't listen to us.
My fiance's mother is a very difficult woman to get along with anyway and she is already angry with me for even suggesting that we have the wedding in October as planned. So I chose to go ahead and postpone rather than upset her any further. If my FFIL wasn't ill, my fiance and I wouldn't back down. But he is already feeling bad enough and he is a good person and I don't want to make him feel worse. If we don't back down, she will nag him to pieces about it and I don't want to put him through that. My fiance feels bad about this. It's not that he won't stand up to his mother, it's just that he doesn't want to put his father through her torture, not while he is sick.
Right now we are planning to wait until he gets the results of his next CAT scan in September. If the tumors have continued to shrink, we are going to set another date within the next couple of months. If they give him very bad news, we will probably wait a little while.
Anyway, thank you again. I do appreciate all of your advice and support and while I do agree with you, I just don't feel I can go through with it if his mother is going to make everyone's life hell.