In my view, this is a case where only engagement has taken place and it is now choice of the husband to take the decision. Unless marriage is not performed, no one can force any of the parties to get married without consent.
Additionally, it would be important to note that this type of decease is curable or not. If the doctor certifies that this decease is curable and nothing in subsequent days of life would get affected again then the groom's family can be convinced about this.
It is expedient that the mediators or elders of both family resolve the conflict amicably else it might get very dirty if you try to get legal assistance. A legal case once sent to a court would fall en-route to final conclusion which may take 3yr-4yr or may be even more. If this is acceptable and you wish to ruin the coming wonderful years of both parties, so be it!
It is imperative that you decide your stance and stand still on that. Don't look back to bad past but look ahead to bright future. There is nothing criminal as such in this complaint and in my view, it would not culminate into desired results for the bride. All the best!