Is it obligatory to inform my family about my fiancée's hereditary diabetes, knowing that they might reject her for this reason?

The selection of a wife relies on piety, character, and beauty, with piety being the most important. If a wife possesses piety, good character, and beauty, then a righteous wife has been found. The mentioned illness is neither contagious nor repulsive, and what is said about its transmission to children is speculative, not definitive. It is not a condition for the family to know about the illness, and there is no sin in concealing it. However, one should not do anything that leads to disagreement with the parents, and if they reject her, then obedience to them is obligatory.